This addition of a second dark color at this stage makes the final black appear more vibrant, and less stark than solid black alone would. It doesn't really matter on this layer what color is used because I will follow up with another layer of black, which when blended with it will prevent this color from standing out and being noticeable. It does need to be dark, though; and using a color that the rest of the tree will later become is a nice guide to go by.
Note: The application of several layers of colored pencil is always accompanied by tiny pieces of the colored wax flaking off, so care should be taken to brush this residue from the drawing, frequently. This is especially important when using dark colors.