Which is a simple mixture for making dark blue?

Correct answer: Blue and a little black

The governing concept is shading: adding a small amount of black lowers the value of a colour and makes it darker. Therefore, mixing blue with a little black produces dark blue, so option B is correct. White would create light blue, while yellow changes blue toward green. Red changes the mixture toward violet or a darker mixed hue, but it is not the simple standard method for darkening blue.