*This is my first guide so it may suck a little bit... Just bare with me lol*
PvP on a Blood Spec DK is a sight to behold... Equip your PvP gear, hit your Blood spec and then lets get going.
Here are a few things you want to remember while PvP'ing as blood.
1. Watch your CD's and make sure you pair them with the other appropriate CD's.
2. Make sure you are not popping CD's needlessly.
3. Please remember that this is not for high damage (although you will have some big crits)... You are there JUST to keep the opposite faction occupied.
Spec: WoW Talent Calculator - Sigrie [33/6/2]
You can change this up as needed but I always recommend getting Lichborne as this is something else you can heal with. This link also shows the glyphs which I feel are best... Again, use your own judgment and adjust to your play style.
The biggest thing you are doing as blood is running in to a big group of people, singling out the healers, and keeping them locked down while your team rapes everything else standing (You may want to duck down if they get that bloodlust look in their eyes). While your damage output isn't great against people... it is more then enough to keep a healer occupied with heals on themselves wasting mana. Also keeping a Dark Simulacrum up on a Mage before they CC or a Warlock before a fear is a huge bonus to keeping the healer locked down. (Yes any spell that uses mana can be cast by you with this. Ring of Frost, Pally wings, etc....)
You also have your interrupts and silence. These are very essential for keeping the healer locked out of their heals long enough that they need to blow their big heals on themselves and when you are going against a healer as blood try to keep as many Necrotic Strikes on them as possible (When that 30k crit heal is healing you... they tend to get pissed).
Once you run in, a lot of people are going to be focusing on you. You want to make sure that your Bone Shield is on you as much as possible. This is a 20% reduction to the damage you are taking. Upon your initial rush in you are probably going to be confronted with a lot of people all attacking you. Icebound Fortitude is great to pop around this initial burst as this is another 50% reduction to the damage you take, as well as Dancing Rune Weapon (if there is a lot of melee) and Anti-Magic Shield.
CD's to pop together when you are low on health: Raise dead, Vampiric Blood, and Lichborne. All of these combined will give you a shit ton of self-healing by performing the following:
Death Pact, Death Strike, Death Coil (healing yourself with it while Lichborne is active). It is very easy to heal yourself from 10% to 100% in a matter of seconds by yourself.
Now that we have the CD's out of the way.... Lets get to the good stuff.
Unfortunately there is no end all macro that I can give you for this. But one thing you have to remember at all times. Disease your healer, spread it, then Icy Chains and Death Grip as needed. Also keep your blood boil on every person you can hit with it as it provides a 10% reduction to the amount of damage they do for 30 seconds.
As a Blood Spec DK in PvP your biggest asset to your team is that you don't take a lot of damage, you are a bitch to CC, and you have a lot of interrupts and a silence.
You will normally keep your Runic power at or near 100 of 130 for the increased damage from Death Strike as this and Heart Strike are your big damage dealers here. The only time you are going to burn this is on your own heals and when you need to do a lot of damage to a healer by spamming Rune Strike.
This is, essentially, all there is to it for BG's. Now as far as using this in Arenas or in world PvP.... It really isn't recommended as you don't have a lot of high damage for big bursts (although it is not bad for 5v5's).
If anyone else has anything you would like to add to this just PM me or post it on here and I will get it added ASAP!
UPDATE: I was completely wrong about 2v2's... This spec DK and a feral druid are absolutely amazing together.