Is "blooming" cocoa necessary in baked goods?
The interaction between the liquid and the cocoa during the blooming process and inside of the cupcake batter during baking is not the same.

During blooming, there is only the liquid and the cocoa interacting in a free-flowing liquid matrix, and in the baking, both liquid and cocoa are trapped in a coloidal matrix. If you bloom before you add to the batter, you make the aromatics in the cocoa more available for the taster. 

Keep in mind that not everyone has the sensory acuity to notice the difference, so it is up to you if you want to invest the time for that