Unless it is a graphic or an illustration, I never use black paint. This painting is such an exception to my rule; I used black and it is not meant to be a graphic illustration image. This is actually a mixed media painting.
Black is the absence of all color and causes a painting to go flat if great care is not used. When used in other media, as watercolor or oil or even acrylic, it will take the life right out of colors, even a small amount. It is great for contrast in illustrations for print however, and in this one I used gouache black and tempra black to get a good flat effect. The difficulty comes in trying to keep any black pigment from getting into any of the colors. So when used, I keep it as a separate paint, not mixed and used last after all other colors are dry. A good bit of prep and preplanning is necessary, but it can add greatly to contrast and effect in an illustration or graphic. I wanted the mermaid to cross from fore-ground to mid-ground across the edge of the black seaweed, so this area was masked after it was painted and before the black was added. Also, the bubbles were masked before the water was painted.
(Well, there are really no rules in painting, just good ideas to use or avoid).