What mainly changes when black is added to the primary colour blue?

Correct answer: Its value becomes darker

Adding black to blue produces a darker version of the same colour family, commonly called a shade. In colour theory, this primarily lowers the colour’s value, meaning its perceived lightness decreases. The blue does not become green or a secondary colour, because no yellow pigment has been introduced. Saturation may change in practical paints, but it certainly does not always increase. Thus option A gives the most accurate main effect.