Those iron on printable patches, do they work on black?

Nope. You can get specific ones meant to be used on dark fabric, though – make sure you get that kind specifically.

Maybe, depends on the design. If it has contrasting edges on everything, or a solid color background field, or all the forms a light shade, the forms would show up but a lot of transfers have clear background and no particular contrast on the edges and could get lost on the black.