Don't underestimate the power of touch-up paint. A good body shop can touch-up and blend a scratch so well you can barely see it.

If you want to do it yourself, mask the surrounding area. With a good brush (i.e. not the brush on the bottle, get a fine brush from an arts and crafts store). Fill in the scratch with layers of paint, allowing each coat to dry in between layers. What you need to do to minimize the appearence is to get the scratched area even with the surrounding area. Let it dry and buff it lightly with some rubbing compound. Blend it with the surrounding area as much as possible and then apply polish and finally wax.

Black is tough to work with, but with lighter colors, you can make even the worst scratches virtually disappear. If you have the money, take it to a body shop. You'd be amazed at what they can do (if they're worth their salt).