Ahmad Black is slow and small, which means it's hard for him to find a role on defense. Too slow to play the deep middle of the field, too small to play linebacker. It's tough to find a niche for a player like that. But his instincts, his understanding of the game and his playing personality warrant him getting on the field. Black was a very, very good college player who overcome significant physical limitations on a team full of top-of-the-line athletes, but it was still college. The NFL is a different beast, and you need a certain level of talent to be an every down player. I don't think Black has that. There is one role in which he can excel, though: as a cornerback over the slot.
In a post-draft press conference Raheem Morris alluded to this possibility: "He is a safety that has the ability to go inside and play corner." Think of Ronde Barber in passing situations, or Charles Woodson for the Packers. They play inside where they can blitz, use their instincts to disrupt plays and where a lack of straight-line speed is not a big problem. Instincts, feel for the game and short-area quickness are important as a slot corner, and those are things Ahmad Black absolutely has. While he may never be a starter, he can be a very valuable and versatile player in nickel packages. And nickel packages are on the field a lot in today's NFL, which is increasingly turning into a passing game. And a good nickelback is pretty good return for a fifth-rounder.