I hope you dont mind but I have a question for you. I just did my floor in my family room (wood) until we get a sectional we have two couches (black) that meet and a corner table. In the middle of the room we have a round coffee table! My boyfriend wants a round inexpensive rug because next year we are going to get new couches, so he doesnt want to invest in an expensive rug if we will be replacing it. Anyway, does it need to be round if the table is or can it be a rectangle and do we need to have black in the rug just because the couch is black? I am sorry, but you are the only one that I know that would be able to help me! I appreciate any guidance! By the way, everything you do is stunning!
As for the color, you don't need to have black in it, but it should be a color that compliments black. For instance, a pastel rug would not be ideal with black sofas. But if the room has artwork with black frames or drapes hung on a black rod, then that will tie the black in well enough.
Don't go with anything smaller than a 6'x9' or it will look chintzy. Better to buy a simpler rug that is the right size than a higher end rug that is too small. It's like couture pants that are 2 inches too short. They may be designer, but they're still highwaters.