What generally happens when black is added to blue?

Correct answer: A darker shade of blue is formed

The governing concept is the effect of black on colour value. When black paint is added to blue, the mixture generally becomes darker and is described as a darker shade of blue, so option C is correct. This addition changes the lightness or value, not the basic hue into yellow or green. It also does not remove blue from the traditional list of primary colours; the mixture is simply a modified version of blue.