OK, so powder coating does not work. Why not use the same kind of color they use on the iPods? I'd go for a bright metallic blue case exterior as long as it had the dull grey inside.
If I'm not mistaken, the silver "aluminum" color you see on all Mac's isn't actually the bare aluminum. It's anodized silver to give it the uniform color you see. If they anodize the MacBook Air's (or any aluminum Mac) black, it would look absolutely stunning. The black iPod Classics are anodized as well, so it would most likely look exactly like that.
Think about it. Every laptop is black. Every. Laptop.
Think about it. Every laptop is black. Every. Laptop.
What do you think, is the idea that Apple is following by NOT making their MacBooks black? They want to differentiate their products from others. As they do with their (often) high quality and their special design.
Black MacBooks wouldn't look bad - but they would look like every other laptop out there. I am not using Apple for Lion only. It's also the thing about its unique design and special, unique look.