Those black rectangles are still likely useable. This is an issue with the mahogany staves as well as Talan points out. They are a 1x2 square rectangle in the inventory that is nothing but a black image. They are hard to see as a result. However, I've used them to make mahogany bows so the seem to function properly. I agree though it is tough to see them in the inventory and can easily be overlooked or not seen at all.
Have you tried making the long bow or short bow with those black rectangles yet? They should still work.
