It depends on what you like and what the rest of your house looks like. For instance, I really like the color red and I could pull off a red rug in my house with a black leather sofa, but that tells me you might be going for a more modern look with black and white pictures, large vases, a nice over-sized chair and a contemporary coffee table. You might want a more country or clean and comfortable look. I would choose a color or pattern you like and then build around that. Currently I have a large square patterned rug and each square is a different color. I try to tell people what my style is and they don't understand, but when they see it they really like it. I love earth tones and color together. Just a quick FYI Target and Home Depot and stores like this also carry area rugs and they are less expensive than a major furniture store. You can buy one there and if you don't like it swap it out.
A rug that goes very well with a black leather sofa would be a tan/black area rug. Depending on the room's color theme you can also go with a gray area rug. To add some color to the room a lighter color rug can be added, such as a lavender or beige. Make sure that you have a few accessories in the room to match the rug that you choose.
If it's black I think you can throw in some color if that's what you like. It really depends on your personal style and what you like best. Black is a great color because it goes with everything. If you don't want the carpet to be colorful than you could do a nice beige and throw in some colored pillows to brighten the room up a bit.