Black is considered a hue (the name of a color) or a value (referring to the darkness of the color). When painting with watercolor, I do not use black paint, but I do in acrylic.
With acrylic paints, the pigments are so flexible and bright. Blacks and grays or lighter pigments and white are used to lighten or darken a value. When I mix two dark acrylic colors together and then try to lighten them with white, they seem to get gaudy. However, if I mix them with a neutral grey # 7 or 5, it seems to be the most effective way to change the value without changing the color.
Black is not a pure pigment, it is a tertiary color (made up of a variety of pigments that create a black value). There are different blacks such as Mars Black, Carbon Black, Lamp Black & Payne’s Gray which would each respond differently when mixed with a color because of the dominant dark hue used to create them.
Watercolor is a transparent media. When you use black paint you don’t necessarily know if it’s more green, blue or what in that pigment. Blacks can get dull when you add water. Therefore, I prefer to think of them as darks. I look at the paints on my palette. To get a good clean dark, I mix the darkest colors (or am more likely to set them next to each other to mix on the paper). This has the added advantage as you can use your dark color to sit back or enhance what it is next to your hue selection.
To understand the dark colors, you truly need to understand color complements. A complimentary color is opposite another on the color wheel. When you mix two complementary colors, they tone each other down. The more equal amount of the complement, the duller or more neutral the colors get. However, if you set them side by side, they bring each other out. The truth is, you can get a dark by mixing almost any two dark pigments… and your dark will have color. By adding black… you are adding one more pigment which could make it more opaque…blocking the light shining through the dark reflecting off the paper… so things get dull in a hurry.
When mixing colors, you could be painting a tan roof (basically from the yellow family) and get a little blue in the mix and it will turn green. You can then neutralize the green with its complement red. When you use black paint…but you may have ability to fine tune a color…but you will need to look for the dominant color and use its complement, because of the unknown matter of the tertiary colors. I prefer to make my darks colorful and no what’s in them.