I've got some pre-made white flower paste, but how can I turn it black?

You can mix in black paste/gel color or you can paint it with a mixture of alcohol and paste/gel color.

If I mix it with Wilton Black gel will it take a lot longer to dry? I assume I'll have to mix a lot in. Should I try mixing any cocoa in to help darken the base color before adding black?

What do you have to make with it?

You could always make it gray and then paint it black.