I almost never use black pigment, except for Payne’s Gray or occasionally neutral tint, and most of the time that is if I’m in a hurry (like with my daily Postcards for the Lunch Bag, often) or if the color just seems to be Payne’s Gray. A lot of animals and birds are that bluish-gray-black color and I can’t see trying to mix it if you’ve already got it.
If I really need ‘black’, I usually use a pen. The rest of the time, I mix it in various ways. I have an ice tray with 3 or 4 mixes of Ultramarine Blue and Burnt Sienna in different concentrations, and often use that, but as often as not, I’ll try to mix black using colors I’m using elsewhere on the canvas.