Blood is going through some big changes in Legion, and tanking Death Knights will be stepping into a brave new world to face off against some demon bastards. Given that my DK is a Draenei, this fight is more or less everything he exists for, so it's pretty exciting.
So where to begin?
Runes of Death:
All Death Knights now have just one type of rune. If an ability would have cost, say, one Frost and one Unholy rune previously, it now costs simply two runes. You still generate ten runic power per rune spent and you can still only regenerate three runes at a time - they just aren't paired up like they used to be.
Blood also no longer has Runic Empowerment or Runic Corruption - you'll simply regenerate your Runes based on your Haste.
Death Strike is Rune Strike:
Blood no longer uses Death Coil. Instead, Death Strike now costs Runic Power instead of Runes, meaning you'll never have to worry about using runes on something that isn't as central and useful as DS. Building up Runic Power also becomes even more important to use your biggest active mitigation ability.
Bones and Marrow:
Bone Shield is now the passive side-effect of one of your new rotational abilities, Marrowrend. Marrowrend costs two runes and does physical damage to the target as well as granting three charges of Bone Shield. Bone Shield reduces incoming damage and also buffs your haste, stacking up to ten but losing a charge any time you take a hit (just like on live.)
Heart Strike is Back, Baby!:
Heart Strike is back. It's a one-rune strike that hits a second nearby target for some physical damage, reducing movement speed of anyone it hits by 50% for eight seconds. This also generates an additional 5 Runic Power, plus 2 for every additional target it hits - which can be boosted by Death and Decay as we'll see farther down. Given the bonus RP, you'll probably want to use this any time you aren't worrying about losing Bone Shield charges, because more RP means more Death Strikes, and this is the most efficient Rune-to-Runic Power ability you have.
Blood Plague Good for Blood Now:
Each DK spec has just one disease to track now, and for Blood it is, appropriately enough, Blood Plague. Blood Plague now drains health to you, instead of simply dealing damage.
Blood Plague is applied in single-target situations by Death's Caress, which has a 30-yard range and costs one rune, dealing a bit of shadow damage up front as well. In AoE/cleave situations, you use Blood Boil. This has no cost, two charges, and a 7-second recharge (affected by haste,) dealing shadow damage to enemies within 10 yards of you and applying Blood Plague.
I wonder a bit if Death's Caress is actually worth using in any situations other than having to infect from range. Blood Boil does more damage and has no Rune cost, so you can save the rune from Death's Caress to use with Heart Strike. Death's Caress might be more of a ranged pulling option than something to use rotationally. Definitely keep an eye on the theorycrafting.
Death and Decay and AoE:
You still have Death and Decay, and now standing in it will cause your Heart Strike to go true AoE instead of simply cleaving onto one other enemy. Blood Boil is still there for snap-AoE, but this will be good for sustaining it.
You lose Icebound Fortitude, but Vampiric Blood is clearly meant to be your major tank cooldown. You also still have Dancing Rune Weapon and Anti-Magic Shell, along with some talented options, so you're going to be ok. And then of course you no longer have Bone Shield as an activated ability, but it's now rotational as part of Marrowrend. Also, Army of the Dead is now Unholy only.
It's baseline for Blood.