Ravening Black Knights at first glance seem pretty expensive, but when you look at the full picture they are well worth the cost and fill a variety of roles. Here's the low down on how to get more out the Black Knights.
Know your Knight
The Black Knight is the veteran form of a Ravenwing bike, so you get an extra attack and +1 leadership over the normal Ravenwing.
Black Knights have the usual Ravenwing special rules such as scout, hit and run, grim resolve etc. In addition they have skilled rider. Note they they do not have ravenwing combat squads or combat squads at all, so if you buy big then you stay big.
Black Knights have the typical wargear though also come with a bolt pistol, corvus hammer and plasma talon. The corvus hammer gives +1 strength and rending, as it just a melee weapon it will give +1 attack with the pistol. The plasma talon is basically an 18" range plasma gun in twin-linked form. At first glance this seems a little short, but when you factor in the Black Knights can move 12" that's a threat range of 30".
You can boost the squad numbers up to 10 in total, squads starts are 3 and is just over 40 points a bike.
The squad leader, the Ravenwing Huntermaster, is basically a normal Sgt who can take melta bombs and a power weapon. The power weapon is 12" points and I seriously wouldn't bother as you're losing that +1 strength bonus. Melta bombs however can come in handy if you have points spare.
For every three Black Knights in the unit one may switch the plasma talon for a grenade launcher. The grenade launcher can fire frag and krak rounds which we're all familiar with. It can also fire rad and statis shells; rad shells do -1 toughness if they HIT and this effects the instant death threshold, though this only lasts for a single turn. I will note that you roll to hit first and then roll to wound comparing the strength against toughness. This is extremely handy as the plasma talons can instagib multi wound T4 models such as Canoptek Wraiths, Ork Nobz, Tyranid Warriors, Tyranid Raveners, Obliterators, Broadsides, Grey Knights Paladins etc. The statis shell does -1 weapon skill and initiative for a single turn, extremely handy if you're going to assault enemy models which are I4 or I5 or have WS5.
Ravenwing Command Squads
While not exactly a Black Knight squad, Ravenwing Command Squads are actually made up of Black Knights and just have a different name. They are also slightly cheaper by a single point and have the ability to take banners. Popular banners would be the standard of devastation which all units within 6" treat their boltguns as salvo 2/4 and the ravenwing company banner which allows units to auto pass hit and run tests. Standard of fortitude is a nice but expensive banner, it gives feel no pain and allows re-roll failed morale and pinning tests to friendly units from the codex within 12".
Tactica
When deploying Black Knights drop them into area terrain as this will give them some form of cover save. As they have skilled rider you do not need to worry about them taking dangerous terrain tests. As they are such a high threat unit they should be really hidden out of line of sight.
If you go first or second you really should scout. This will allow you to re-position your force to avoid enemy fire power, take cover and also get closer. Ravenwing is a short ranged army and you do not want to be at the back of the table getting shot at and then take another turn of getting shot up again.
Generally Black Knights should be in single shot range first turn, if you deploy 12" then scout 12" and then a 12" movement that's a whooping 36" before the game has started. You will be out of double tap range, but better to be shooting something than nothing. If you cannot shoot then turbo boost, skilled rider gives +1 to jink saves so the Black Knights will get a 3+ cover save when using turbo boost or 4+ cover normally.
Thanks to their speed combined with scout move you should be able to position your units to get into side armour or at least setup for assault next turn. Remember, if you scout you cannot assault first turn if you went first.
Black Knight's purpose in a list is a sort of half finished deathstar, they are just as survivable as normal Ravenwing but pack a lot more punch. Their wargear allows them to be extremely flexible in choices of target, they can hunt monstrous creatures, elite infantry and are fine at tackling camping armour, transport vehicles and decking bog standard troops. They may struggle against dedicated assault units, but if you manage to get some plasma talon shots in support with rad shell and/or statis shell then you should be ok. Another type of force they may struggle with is hordes, but bolter salvos from supporting units along with hit and run should mean they shouldn't get bogged down.
A unit of 5 x Black Knights cost 210 points, which I'll compare against equal costed units. The Black Knights will carry a single grenade launcher. All examples will presume Black Knights will be shooting, after all they should have the initiative due to their speed and scout move which will allow them to pick their targets.
Black Knights vs Hammernators
Grenade launcher fires, stands 50% chance of scatter with average 3" scatter. If positioned correctly it should still hit. Presuming the Hammernators are spread 2" apart the blast should hit a single Terminator. The rad shell stands 50% chance to wound. More importantly the Hammernators are now T3, this doesn't effect plasma shots, but will aid the Black Knights in close combat.
Black Knights fire 8 plasma shots - hit 7.08 - wound 5.94 - 2.01 dead Hammernators
Black Knights charge and get hammer of wrath, they score 3.3 wounds which is 0.53 dead Hammernators.
The Black Knights will strike first, they will get 4 attacks each giving them 20 attacks - hit 10 wound 6.6 wound, 1.65 should be rends. Hammernators will fail 0.56 rends and fail 0.94 armour saves - that's overall 1.5 dead Hammernators. On average this will leave a single Hammernator left, who would kill a Black Knight.
Black Knights vs Tactical Marines
To balance things out lets say the Tacticals have a plasma gun, missile launcher and the Sgt has a power axe. To keep things a little more fair I am going to presume the Tacticals are in 5+ cover, after all no one is going to leave them out in the open with all this plasma about unless their ride gets wrecked.
The grenade launcher will again fire rad shell. Against Marines the stasis shell has little use as enemy Marines will still hit on a 4+ while the initiative will mean they will strike after. The -1 to toughness will mean the Black Knights will wound on a 2+, which is a lot better. Due to the numbers of the Marines lets say the rad shell hits three, it will wound 1.5, which is 0.51 dead Marines.
Black Knights fire 8 plasma shots - hit 7.08 - wound 5.94 - 3.98 dead Marines after cover saves.
Black Knights charge and get hammer of wrath, they score 3.3 wounds which is 1.12 dead Marines.
Tacticals will overwatch with 5 bolters, will hit with 1.6 and wound 0.53.
Knights will get 4 attacks each giving them 20 attacks - hit 10 wound 6.6 wound, 1.65 should be rends. Marines will fail 1.69 armour saves - 3.34 dead Marines.
The Marines get to strike at the same time though, there is 6 left thanks to the plasma shots including the Sgt.
Marines get 5 attacks each - hit with 2.5 and wound 0.83, Black Knight stands 0.28 chance of failure. The Sgt will attack with the power axe with 3 attacks, will hit with 1.5 - wound 0.75 - lets say a dead Black Knight.
Overall the Black Knights have killed 7.83 Space Marines while taken a single causality themselves.
Black Knights vs Trygon
In this example I've used a Trygon to see the effects of the statis shell. Again, will be presuming it will be hitting as 50% chance to hit and average scatter will be 3", the Trygon after all is on a super size base. So now the Trygon will be reduced to WS4 and I3. This is massively important as the Trygon is a Black Knight killing machine and having that strike last will mean it will be taking a beating before it even strikes. Like other examples, I will presume the Trygon is in cover, after all no one would leave super snake out in the open! I'll also give the Trygon toxin sacs so it is exactly the same points cost.
Black Knights fire 8 plasma shots - hit 7.08 - wound 4.67 - Trygon takes 3.21 wounds.
Black Knights charge super snake and hit 5 hammer of wrath, will wound 0.64 and the Trygon will fail 0.22.
Knights will strike first thanks to the handy statis shell, they will get 20 attacks - hit with 10 - wound with 3.3, which 1.65 will be rends. Trygon will fail 0.56 armour saves.
Currently the Trygon has lost 5.42 wounds (I didn't count the hammer of wrath), so it is barely alive, but still ready to kick ass. The Trygon gets 6 attacks - hits with 3 - 2.65 Black Knights.
Next turn the Black Knights would snuff it and would only take the Trygon down if they got a lucky rend.
While not as effective against a mega wound monster, the Black Knights have done some major damage and it is was an 'average' monster which has only 4 wounds i.e Hive Tyrant then it would probably be dead.
Summary
As you can see Black Knights are an extremely flexible unit and can deal with an all manner of targets. Sure they struggle against super mega tough monsters, but who doesn't? Think of them as a a deadly fast moving dakka unit who can finish most things off in assault.
Of course Black Knights are not without their problems. Dedicated assault units will munch through them if they get the charge. Horde armies will bog them down if the Knights do not manage to hit and ruin and use weight of attacks to bring them down. And they die just like normal Ravenwing.
Do you use Ravenwing Black Knights in your army and if so how do you use them?