Prior to patch 3.1.0, Blood Boil did no damage to targets with no diseases on them. Hence one would first stack the target with diseases, then use [Pestilence] to spread them to nearby mobs and then use Blood Boil. Until patch 6.0.2 this was still an effective method of boosting Blood Boil's damage, however it could still be used to do some damage even to undiseased enemies. Using Blood Boil in this fashion was primarily useful when tanking or for killing swarms of low-health targets.
Tips and tactics[]
As a tank, Blood Boil is very useful for picking up stray mobs while your other AoE abilities are on cooldown.
Patch 9.0.1 (2020-10-13): Now learned at level 17 (was 56).
Hotfix (2016-11-14):Blood Boil now deals full damage in PvP (was reduced).
Patch 7.0.3 (2016-07-19): Redesigned as a Blood-only ability. Now deals damage to nearby enemies and infect them with Blood Plague. Blood Plague now deals 82% more damage, drains health, and lasts for 24 seconds (down from 30)
Old: Deals Shadow damage to all enemies within 10 yards, and spreads your existing diseases from your target to all other enemies hit, refreshing them to full duration.
Patch 6.2.3 (2015-11-17): Damage has been increased by 10%.
Hotfix (2015-01-12):Blood Boil's damage has been reduced by 33.3%.
Hotfix (2014-10-15):Blood Boil should no longer incorrectly grant Scent of Blood or Runic Power if it doesn't strike a target. Additionally, Blood Boil should no longer incorrectly cause diseases to spread back to the casting Death Knight.
Old: Boils the blood of all enemies within 10 yards, dealing X Shadow damage. Deals 50% additional damage to targets infected with Blood Plague or Frost Fever.
Patch 5.0.4 (2012-08-28): Now learned at level 56 (was 58).
Patch 3.1.2 (2009-05-19): If no target is available for Blood Boil to hit, the spell will now cast and consume a blood rune but generate no runic power.
Damage increased to make up for [Pestilence] no longer doing damage.
Now does some damage to undiseased targets and extra damage to diseased targets. Its radius has been decreased.
Old: Causes any of the Death Knight's diseases on the target, and all enemies within 30 yards of the target, to painfully erupt, dealing X Shadow damage.