LEFT: Ivory Black drawn out with titanium white. RIGHT: Chromatic black drawn out with titanium white. In this sample, the chromatic black is warm (red shade) until drawn out or mixed with white, where the green is dominant.

Impressionist painters avoided using straight black in the shadow and dark areas of their paintings because straight-from-the-tube black is a rather lifeless color. They chose instead to use chromatic blacks (a.k.a. mixed black), which usually don’t contain any black pigment. They are mixed either on the palette or in advance from two or three complimentary colors: a dark green and a dark red, or a dark blue + dark red + earth color (a brown). Chromatic black provides the needed dark value while being a more interesting color. Chromatic blacks can be warm, cool or neutral, depending on how they’re mixed.

Here’s a good article on the subject on Gamblin’s website.