Midnight blue is a profoundly dark blue. It gets its deep, dark tone from a healthy addition of black. In terms of decor, it's probably a color best used with restraint, either as an accent or on trim or similar smaller areas. A midnight blue room would feel oppressive, and the effect of the color when painted on the four walls of a room would far exceed the impact of the paint swatch. Think of it primarily as a contrast color. If you decide to use midnight blue on the walls, using it on a single accent wall is one way to introduce the color without overpowering the room. Use blue paint and black tint, which is more concentrated than black paint, to mix this color.
Open the dark blue paint. Start with an ultramarine or similar pure blue, rather than a sky blue that already has white mixed in, preventing you from getting a true midnight blue. Pour the blue paint into a mixing container if you are mixing less than a gallon of paint.
Add a small amount of black tint from the tube. Start with no more than a teaspoon.
Stir well, using a paint stirring stick or a drill with an attachment for blending paint. Keep going until the color is uniform.
Evaluate your color and decide if you want to go darker. How deep and dark the midnight blue turns out is up to you. Remember, the paint will dry a hint darker than it looks when wet.
Continue adding tint, mixing well each time, until you reach your desired shade of midnight blue.