when does karrablast evolve in pokemon white?

Karrablast does not evolve upon reaching a certain level. When traded for Shelmet (#616), Karrablast evolves into Escavalier (#589) and Shelmet evolves into Accelgor (#617). These are the only two Pokemon that must be traded for each other in order to evolve.

Sunday, February 05 2012
Source: http://bulbapedia.bulbagarden.net/wiki/Karrablast