Make blue paint darker by tinting it with black paint; adding a color to another color to create a variant of the second color is called tinting. Black darkens the paint. Both the blue and black should be the same kind of paint--for example, mixing acrylic and tempura paints won't work because the mix will not dry correctly.
Things You'll Need
64 ounce bowl
Pour blue paint into the bowl. Fill about three-quarters full.
Dip the tablespoon into the black paint and fill. Pour the paint into the bowl. Do not let the tablespoon touch the blue paint in the bowl.
Stir the black paint into the blue paint with a clean paint brush. Mix the black paint into the blue paint so there are no black streaks.
Wipe a streak of the tinted paint onto a piece of paper to see the color. If you want the paint darker, repeat the process with another tablespoon of black paint.