I’ve had a tube of ivory black watercolor for several ears. I bought it because it’s the least opaque of all the ready mixed blacks that come out of a tube. I barely ever use it though, because when it dries it looks flat, which is boring and unappealing. It’s also a staining color so it can’t easily be lifted if I make a mistake. The only time it comes in handy is when I’m painting portraits, and I mix it with cadmium red when painting eye lashes.
I’ve found that mixing my own gives me a lot more flexibility. I can mix a warm or a cool black as well as a reddish or greenish one, The best way to mix black is with ultramarine blue and burnt sienna. It’s varied and interesting. I can make it a warm black by adding more burnt sienna, or a cooler one by adding more ultramarine blue. The two colors have a granulating effect that catches the light in appealing ways.
The best thing about this mixture is that these two colors aren’t staining. That means the pigments don’t get down into the fibers of the paper. They sit just on the surface and can be lifted easily with a wet brush and a paper towel if I mess up. Even after it has dried!