DKs in general:
Outlast them, especially blood. They will heal if they can hit you, which simply means don't get hit. Start with a stun, then gouge, then dismantle them. Then stun them, maybe another gouge, and evasion. At this point you have nothing, and I'm sure they have immuned a stun, so DRs are up. (If they trinket or immune one of these just move on, you get the idea) Once everything is used though, sprint away, and try to get a smokebomb on your location before they grip you, to avoid said grip. Reenter stealth some way or another, let DRs finish, and repeat! Fact is DKs can't put out much damage while healing themselves, and you can heal through that. If you can outlast a DK while not getting hit, you will win. This might take a while, but it is much less futile than hitting a healer. Still, if you are in a BG or arena, don't bother with a blood DK, tanks don't do damage anyway.

Ferals:
These are considerably tougher. You want to open on them, since you need every advantage possible. They have no way to remove stuns aside from trinket, and they need to go out of combat to heal. These are really their only disadvantages, so use em. Anyway, try to keep the feral in combat. You don't want them restealthing, and since they have faerie fire on you, you can't restealth without CoS. Try to save CoS for when they root you and go to heal. Then vanish and get to them (SS if sub). FERALS HAVE AN ENRAGE. Memorize what it looks like, i believe it creates a red glow on the paws, and the buff is the head of a cat-like animal, but I could be wrong. When you are low on energy, just kite them. keep them in combat via throw, but there is no sense in fighting if you can't use any abilities. Also, when they cyclone and heal, that is when you trinket.

For both, control is your greatest advantage, so use it. Don't get hit unless you are hitting them harder, keep up recup (esp if you are sub), and force trinkets or immunities before bursting them.