Using an online color mixer at Trycolors.com, it appears that if you mix the color blue with the color black you will get "navy blue". As per the The Printing Art Magazine, navy blue is sometimes mixed from four parts of "black" and one part of "ultra blue".
Color mixing is something beginners often shy away from. Don’t, rather learn the few fundamentals, embrace the challenge and get mixing. At worst you’ll produce mud colors; if you don’t want to waste the paint by throwing it away, use it with some white to do a tonal exercise, or underpainting.
Check out some tips to help you get the best results when mixing colors at About.com.