I had a couple of small decals on my old vehicle for at least a year and the only thing I noticed was when I removed them there were very light scratches in the paint along the edges of where the decals were. After I waxed over the paint you couldn't even see them. Before I put them on there I de-waxed the area so they would stick well and they did. They were just as easy to remove too.

I would say on a black car something bold would stand out like white, red, yellow or orange. I'm thinking about getting this Mitsubishi window decal for the back that looks cool. I can't remember what website I found it at but it's the 3 diamonds with flames coming out of them. The only problem I think I might have is not being able to see well out of the back window once I put it on. It's hard enough as it is when I look out and all I can see is my spoiler.

 

You gan get back window Decals that are cout our of a special kind of vinyl that is semi transparent. So you can see out of them decently. But do not de wax the area under the vinyl. Make sure it has a good coat of wax before and is very clean. Then wax after is has been baked on a few very sunny days before scrubbing and re waxing.