Half black guppies are nice . Just their heads arent black. I also agree with the balloon molly suggestion. I have some and they dont bother my platies, guppies or Angels. Fish are like people . Most of them are good but some are just plain jerks!
One male black Molly (non-balloon, non-sailfin type) would be my choice. I have a jet black one in my 15G tank along with two other mollies and a fantail goldfish. He bothers no-one and keeps to himself. He's adorable. Go get one today...
I wish I could remember the full name, but there are some solid black Guppies that are really nice. There were a number of them available at our clubs Fall Auction.
I have a nearly black betta, with black fins, but he has a gorgeous blue shine to his body. (in my profile pic, I keep meaning to change it because it's hard to see him, due to his darkness) I'd go with a betta or black molly. I bet it will be a pretty tank!