Black vs. Blue

Black is a self color with color genetic make up of:

aaB-C-D-E-
 

Blue is a self color with color genetic make up of:

aaB-C-ddE-

Comparing the two, the difference is in the d location.   The black has D that stands for "dense" color and the blue has dd that stands for dilute color.  Blue is the dilute of black.      Both black and blue carry E, thus the black/blue color are extended to the rest of the body.