Mind Blast is a key part of any shadow priest rotation, and should generally be used on cooldown. As well as dealing substantial damage, it is critical for generating insanity for use with [Devouring Plague].
An exception is while during [Voidform]. Devouring Plague and [Void Bolt] should be prioritized over Mind Blast.
Patch 10.0.0 (2022-10-25): Rank 2 removed for both Discipline and Shadow.
Patch 9.0.1 (2020-10-13): No longer exclusive to Shadow priests and now available at level 5 (down from level 10). Cooldown increased to 15 seconds (was 7.5 seconds). Mana cost added. Insanity generation reduced to 8 (was 12) for Shadow.
Rank 2 added for Discipline (Level 18, causes Mind Blast to prevent next (300% of spell power) dealt by the target) and Shadow (Level 13, reduces cooldown by 7.5 sec).
Patch 8.0.1 (2018-07-17): Insanity generated reduced to 12 (was 15). Cooldown reduced to 7.5 seconds (was 9 seconds). Now benefits from [Mastery: Madness].
Mind Blast damage increased by 30% in PvP situations.
Patch 7.1.5 (2017-01-10): Now generates 15 Insanity (was 12).
Patch 7.1.0 (2016-10-25): Now learned at level 10 (was 16).
Patch 7.0.3 (2016-07-19): Damage increased by 50%. Mana cost removed. Now generates Insanity instead of [Shadow Orbs]. Now learned at level 10 (was 21).
Patch 6.2.2 (2015-09-01): Now deals 10% more damage. In PvP combat, Mind Blast now deals 10% less damage.
Patch 6.0.2 (2014-10-14): Cooldown is now reduced by haste, but its base cooldown is now 9 seconds (up from 8 seconds).
Patch 5.0.4 (2012-08-28): Now a level 21 Shadow specialization ability (was a level 9 ability for all priests), and generates one Shadow Orb. Spell visual animation changed.
Patch 4.2.0 (2011-06-28): Mind Blast damage done has been increased by 12%.
Patch 4.1.0 (2011-04-26): Casting an instant Mind Blast immediately following a [Mind Spike] should no longer cause Mind Blast to not benefit from the last cast of Mind Spike.
Patch 4.0.6 (2011-02-08): Now does 18% more damage than Mind Spike.
Patch 4.0.1 (2010-10-12): Now learned at level 9 (was 10).
Patch 2.0.1 (2006-12-05): No longer generates bonus threat.
Patch 1.9.0 (2006-01-03): Threat raised on ranks 7, 8, and 9. Incorrect data had been entered in those spells, resulting in less threat generated than designed.
Patch 0.9 (2004-08-17): Now has a short cast time.
[Apathy], 
[Dark Indulgence], 
[Expiation], 
[Harsh Discipline], 
[Inescapable Torment], 
[Insidious Ire], 
[Manipulation], 
[Mastermind], 
[Mind Devourer], 
[Shadowy Apparitions], 
[Void Summoner], 
[Whispers of the Damned]
[Holy Fire] replaces Mind Blast at level 19 for Holy priests.
[Voidform] is active, Mind Blast gains a second charge, and its remaining cooldown is refreshed upon activation.
