Almost all upper & lower recievers are finished by anodizing. This is an electrochemical process that turns the outermost aluminum atoms to aluminum oxide, AKA alumina, which is a hard but porous ceramic. This is a clear coating that can be dyed a variety of colors, including black. Black is one of the hardest colors (or lack of) to make in a dye. Most are really very dark greens or blues. If the dye is off a tiny bit, the bluish color will show. After the dye job, the anodizing is sealed with an acid bath and no adjustments to the color are possible. The part is wet throughout the anodizing/dying/sealing process so the tinge of color would be hard to detect.
These minor color variations are normal, blue/purple, green, reddish brown. The alternative is to scrap these receivers, dump the bath & make more. Needless to say, this would greatly increase the price of the guns we can buy.
Nothing wrong with your Bushmasters, at least in the color. Hang on to 'em, shoot 'em & enjoy.
Norm