i am trying to make a birthday cake and i want to use black fondant. but my question is when using a stencil what is the best to use airbrush or paint?

I like an airbrush much better when stenciling.  I find there is less bleeding.  Also, I apply a thin layer of shortening to the bottom side of my stencil so it adheres snugly to the cake.  That helps.

Typically people use royal icing for stenciling