In the painting in my latest post I haven't used any black. Instead, to get a really dark color Ive mixed ultramarine with burnt umber.
There are various ways to mix dark colors, this is just one.
This kind of black will of course not be 100% black and that is what I love about it. You will be able to see brown and/or blue reflections in it, depending on how much blue versus brown you use when you mix them together. In the shadow on the arm I let the brown be dominant, and for the dark parts of the skirt, I've used more blue.