Likely you may have heard this in art school, an art class, or from another instructor. In my world, there is no such thing as "don't use black" because you need black in order to make tones (black plus white). Whether you choose to add black to a color, or to white, and then add the gray mixture to a color, there is really no reason not to use black paint. In my watercolor days, black was never on the palette. There were easy ways to achieve a dark almost as black as black. But, there is no one in this world who can convince me not to use black. No rules when it comes to color. If you like it, use it. . ;) :)