You can have a black background and not have it be low key. I find that black works better with most subjects than white does.
With white you can get several lighting issues, one being your background is competing for attention with your subject because the eyes do go for the lightest area most of the time. Another is the light reflecting back from a white background (sometimes called blow back) can cause loss of contrast and even lens flare. If your subject has dark clothing on or is dark skinned, you can create a higher contrast situation than you might want by using a white background. And if you don't light it correctly, a dirty white background can detract from the image.
There are lots of times where a white background is appropriate or a good idea, but there are also times when it isn't.
Black absorbs light rather than reflect it, so you have less of a lighting issue. Although it can create the opposite high contrast situation than what white creates, black seems to be less of a problem sometimes than white would be. And it's easier to change it's tonal shade with lighting and not have it look dirty or muddy than white is.
I find that I used my solid black muslin more than I did any of my other solid color muslins. If you are going to get a black muslin, make sure you get one where the threads have been dyed black before the muslin was created. This gives you a solid black. A muslin that is dyed after is likely to have splotches and can fade in worn areas.
Although there are a number of sources, I have used a 9' wide by 18' long black muslin (called Commando Cloth) that I got from Chicago Canvas years ago. It's still holding up pretty good. I did have to seam up the side edges when they started fraying after washing and drying it about bunch of times. When I wash a dyed muslin like this, I dry it till it's just damp. I then hang it up on my background stand and let it finish drying. It comes out wrinkle free and smooth.
For solid colors, black, blue, white, red, are good choices. Other than a couple of painted muslins, I still use the black the most though.
Mike