I have just airbrushed a cake black which looked great (even though my nose didnt!) but now the cake is starting to dry it has a bumpy sort of finish like tiny tiny little airbubbles.
What did I do wrong?

what did you frost your cake with?

Fondant

The black color..or any airbrush color brings out the texture of whatever you are airbrushing so if there is flaws...they will show more...If the fondant is bumpy or pock marked with tiny pin head holes it will show.It is not the color unless the airbrush is spitting color and then you have to clean or replace the needle.