I'm going to suggest neither, or a contrast. 
I've used blue background with black substrate, and I really didn't like the way it looked. Too much like an LFS tank, and it didn't do it for me at all.
I've also used black backgrounds. They look nice in planted tanks where the plants cover most of the background, but black substrate and backgrounds will make a lot of fish go dark.
I used to support the advice that they make fish more vibrant but not anymore. I believe a compromise is best, black substrate with no background (white wall) looks REALLY nice to me.
So do black backgrounds with light substrates like white/beige sand. (see my avatar for an example)
When you have a combo such as those, that's when I've seen amazing contrast and color in my fish. (quality food and clean water play a heavy role as well, of course).
HTH

I've used blue background with black substrate, and I really didn't like the way it looked. Too much like an LFS tank, and it didn't do it for me at all.
I've also used black backgrounds. They look nice in planted tanks where the plants cover most of the background, but black substrate and backgrounds will make a lot of fish go dark.
I used to support the advice that they make fish more vibrant but not anymore. I believe a compromise is best, black substrate with no background (white wall) looks REALLY nice to me.
So do black backgrounds with light substrates like white/beige sand. (see my avatar for an example)
When you have a combo such as those, that's when I've seen amazing contrast and color in my fish. (quality food and clean water play a heavy role as well, of course).
HTH