Black and blue are combined to create navy blue. Because navy blue is a dark variation of blue, which is a primary colour, adding black to blue produces the navy hue.
Colors are created on computers by combining red, green, and blue lights. A value between 0 and 255 is used to denote the level of light intensity utilised to create each of the three hues.
Then, the intensity of each light colour is used to indicate the new colour. For instance, black is 0,0,0 while white is 255,255,255. With intensity levels of 0,0,128 navy blue is created.