If you look at the darkest blue in the shades strip you'll agree that the blue has lost its sense of blueness and become somewhat muddy and murky. Other times, adding black to a color can have some unexpected results. In the example above the yellow center square is the pure color. The olive green color was created when a touch of black was added to the yellow. The darker yellow was created by adding purple - the complementary color of yellow.
Color complements are color opposites.
If you look at a color wheel , the blue arrow is directly opposite its complement color, orange. Adding a touch of purple to yellow or blue to orange will darken the hue without the muddiness that black can cause. A color wheel is one of the best investments that an artist can make in reference materials. Features include primary colors, tones, tints, complementary colors, and harmonies. Well illustrated with examples of these concepts.