Improved Mind Blast is a priest talent that reduces [Mind Blast]'s cooldown by 0.5 seconds per rank, for a maximum reduction of 1.5 seconds, leaving a 6.5 second cooldown. In addition it gives Mind Blast a 33% chance per rank to reduce all healing done to the target by 10% - up to a maximum 100% chance - while the priest is in [Shadowform].
Mind Blast, though one of few ways for a Shadow priest to deal burst DPS, is very mana inefficient compared to Mind Flay and DoTs. With that in mind, this talent is geared more toward PvP, where burst DPS is king - a shadow priest looking to maximize his sheer kill power in PvP will find this talent very useful. It's much less useful in PvE, however, where Mind Flay and DoTs will make up most of the priest's damage.
Patch 4.0.3a (2010-11-23): Now reduce healing by 10%, down from 25%.
Patch 4.0.1 (2010-10-12): Reduced from 5 to 3 points and moved from tier 3 to tier 2. Cooldown reduction at max rank reduced from 2.5 sec to 1.5 sec. Healing reduction increased from 20% to 25%.
Patch 3.2.0 (2009-08-04): Redesigned. Now, in addition to reducing the cooldown of your Mind Blast spell by .5/1/1.5/2/2.5 seconds, while in Shadowform your Mind Blast also has a 20/40/60/80/100% chance to reduce all healing done to the target by 20% for 10 seconds.
[Mind Blast]
[Shadowform].
Patch 5.0.4 (2012-08-28): Removed. 
