In graphics work, when I need to present multiple graphic image choices to clients, I spraymount the images to black mattboard and leave a border of the black matt to show 3-5 inches around. The black helps set these lighter images I've spraymounted apart from table tops, walls, and whiteboards that the images are shown against since there is alot of white and bright light reflecting in an office setting.... and the black works great for this use.
But, I haven't used black for matting in my own personal fine art work as it wouldn't be appealing visually, and even if I create vibrant brilliant colorful abstracts I know I wouldn't use black because it's too heavy a feel for me personally. In fact, I think I'm getting away from matting, going for the no matt look. Just my two cents, not advice!
I also agree with Peggy's thoughts....a light matt with a thin black frame is classic and classy with an abstract as well as pen and ink, contrasty types of artwork.
I understand your roommate's thinking tho'....black as a background matt does push an image forward. You gotta really be happy with your matt so it's an entirely personal choice.