Blackfury is one of the top pre-BC polearms in Azeroth. It is created by Blacksmiths and gives bonuses to strength, stamina, and fire resistance.
Blackfury is crafted by Blacksmiths with a skill level of 300. Prior to 2.0.3, it also required the Weaponsmith specialty.
The plans are sold by Lokhtos Darkbargainer in Blackrock Depths. You must have revered reputation with the Thorium Brotherhood to purchase these plans.