whell since Blue is a primary colour i guess you can add black? perhaps.
chocolate!
Pink and a dark dark Blue
Dark purple.
Blue and black
light and dark blue. ok?
The two colors mixed together to make dark blue is blue and black. You can make any color darker or lighter using black and white.
When you mix red and blue, you get purple; for a lighter shade, you just need white, which the icing itself would probably supply. you start with white icing....... then add purple icing a little at a time...... to make purple, you mix red and blue. more red than blue though. If you use too much blue, it turns black......... now, the lilac icing will turn darker the longer it sits so you want to make it, ice the cake, then serve it right away....... It will turn the prettiest shade of dark purple the longer it sits.....
dark green and white or yellow and blue
My best guess would be blue and black.
The colors you mix with sky blue to get navy blue would be a dark blue or a black. These will make a navy blue.
Dark purple, blue, and green
Some combination of dark blue and black paint.
Using white icing you will have to add blue food coloring to it.