Blood is a tanking spec that uses abilities to regain and take damage awhile keeping good threat with decent damage. It is very good at soloing quest that you will sometimes need 3-5 players on. You can also kill 2 or more targets with its good damage taking abilities and healing. Also really good for gearing up because of instant ques in to heroics. Blood tanks are some of the best that the game has to offer because of there awesome health regaining ability. before i am going to go in to detail with what the blood spec is im going to give you some back round

Bloods Background

In WotLK days Blood, Unholy, and Frost could all be decent tanks but The Talents were always complicating to deal with because also each spec could be a dps. When there was 3 different trees for tanks I still used blood because of its healing and damage taking abilities. When patch 4.0 came out it is like the blood that it is today the only tank spec. Some peoples opinions to it was that they preferred frost or unholy as there tank spec and did not want to use blood in 4.0 but they really had no choice. Back in WotLK days there was also an option to do Dual Wield DK tank which were good but not my cup of tee.

How to use Blood

if you specialize your talents in the Blood tree, your style of play will have two defining features. The first is that your main attack, Heart Strike, uses only one rune. This will result in you doing more attacks, albeit with less strength. The second is that, in comparison to the other trees, the majority of your damage is physical and not magical.

Blood Basic Build
Basic PvE Leveling (52/0/0)

Example of useful rotations:A Blood Death Knight will rely on physical damage while having above average self-healing abilities to improve survivability. This is more obvious when investing in Vampiric Blood and Rune Tap, and as you level up Improved Rune Tap. You will gain expertise and strength bonuses by investing in Veteran of the Third War, Abomination's Might, and Blood Gorged; however, a Blood Death Knight will shine the best when combining his Death Strike, Rune Tap and Vampiric Blood with Blood Gorged by keeping himself at full health the majority of the time.

Blood Rotation

Plague Strike -> Icy Touch -> Heart Strike -> Heart Strike -> Obliterate -> Death Coil -> Plague Strike -> Icy Touch -> Heart Strike -> Heart Strike -> Heart Strike -> Heart Strike -> Death CoilIcy Touch -> Plague Strike -> Death Strike -> Heart Strike -> Heart Strike -> Death Coil -> Obliterate -> Icy Touch -> Plague Strike -> Heart Strike -> Heart Strike -> Heart Strike -> Heart Strike -> Death Coil -> Obliterate -> Death Coil

Suggested Major Glyphs

Glyph of Death Strike
Glyph of Vampiric Blood
Glyph of Dark Death

Suggested Minor Glyphs
Glyph of Pestilence
Glyph of Blood Tap

From here, filling out the Blood Tree is a decent way to proceed, with Bloodworms, Vendetta, Scent of Blood and Improved Rune Tap all being strong talents for soloing. Picking up Virulence will give an advantage in Player versus Player combat, especially against dispelling classes (if that's an interest of yours), as well as a small improvement to Death Coil.
Blood Raid build Edit

Other Build

51/13/7 This build focus's on Obliterate & Heart Strike as main dps, ignoring Blood Strikes & Death Strike. Improved Death Strikes is not taken. Glyph of Obliterate, Glyph of Icy Touch & Glyph of Death & Decay are taken as Majors to increase the effectiveness of Improved Icy Touch & Black Ice. Bloodworms adds a negligible amount of dps, but with no DS spending for improved DS is a greater waste.

Blood PvP build

While most of the core talents of a PvE Blood build is still useful in PvP, PvP Blood Death Knights need several more talents to enhance their survivability from enemy players and help to defeat them. These include -
Heart Strike & Glyph of Heart Strike - While the other trees have talents to reduce your enemies' movement speed, Blood has a glyph. Being a melee class, slowing your enemies is essential.
Rune Tap & Improved Rune Tap - Rune Tap is considered by many to be an essential PvP talent for all Death Knights regardless of their main tree. While other trees have to dig deep to fully upgrade this ability, as a Blood Death Knight you can max it easily, and should do so.
Mark of Blood - can effectively neutralize fast attackers like rogues, feral druids, hunters and fury warriors and will severely hamper the damage done by others.

Most users wouldn't use Blood for PvP, mainly because you are easily kited around. Personally I would disagree, Blood can be kited but with the right rotation on CD's you should be able to get back in. Blood has a very high ability to stay alive with its own heals like Vampiric Blood, Rune Tap, and Mark of Blood. Blood is a rogue and feral druid killer, since they really cant do anything to you with your constant heals and wearing plate. It's not good to spec into full Blood for PvP (trust me, its not good) and Unholy would power up your spells and diseases like Death Coil and Icy Touch. So, to get the most out of your melee, the only option really is going hybrid to frost. Speccing down to Annihilation would be your best bet, since you can burst it with Empowered Rune Weapon without having to worry about losing the bonus damage from your diseases. Making the choice from either Killing Machine or Runic Power Mastery, you'll either get 6 secs on your Dancing Rune Weapon, or you can auto-crit with Icy Touch if they get out of range, which does make a difference. From there, stop putting points into Frost and focus back on the Blood tree. Spec all the way to Dancing Rune Weapon for the instant death on someone, and make sure to pick up Vampiric Blood and Hysteria. IMO stay out of Blood Worms because Blizzard has yet to make them useful, and stay out of Vendetta because it's not worth 3 points for something that has a 5% chance on keeping you alive, if you're dealing with a caster or pally. So - big hits, melee hits, short range, and heal yours