Black face is a new mutation in which the black stripes (undulations) of the head extend all the way into the face and mask, as well as the body feathers. The blackface mutation also causes a darkening of the body color. This mutation is extremely rare and last known to only exist in the Netherlands.

Basic Genetics:
Normal - dominant
Blackface - recessive

For more info on blackface budgerigars and their genetics, see The Blackface: a New Mutation in the Budgerigar by H.W.J. v.d. Linden

photos courtesy of and copyright Didier Mervilde
The blackface variety's undulations extend into the mask and body feathers.