Black masks improve vision in low light by increasing contrast. They block light coming into the sides of your eyes, and it is easier to discern subtle diferences in shading. The effect is similar to driving into the sunset, and shielding your eyes with your hand.
This is most useful during dives at sunset, or night dives, or deep dives, all where light is low. They are also very popular with underwater photographers, who get big gains in seeing the viewfinder.
Clear masks give you a nice, spacious feeeling, so it's good for claustrophobes. It also increases your peripheral awareness, because although you can't truly see through the sides of the mask, you can see shapes and movement. The cost is a lot of glare, and when you know to look for it, you'll see the reflection of the side of the mask in the faceplate.
<shrugs> It's a tradeoff, both designs have their advantages. As a generality, divers typically start off with a clear mask, and as they pick up a lot of experience, switch to a black one.
Personally, I have a copy of both. I'll use the clear when I have students and want to be very situationally aware of my surroundings, and use the black for everything else.
All the best, James