The black color comes from dye added during the Hard-Coat Anodizing process.  Very small variations in metal formulation, heat, current, and/or dye can affect how well the color is "taken" during the process.  Black is one of the hardest colors to make.

If you've ever dyed anything black, you know that if the dye is dilluted too much, or doesn't take to the material well, it will look dark purple.  That's what happens with some receivers.  It is usually more noticable in white light or when oiled.  It is only a cosmetic issue, not a functional one.

Although this problem has been observed occasionally in receivers from every company that uses the Mil-Spec HCA finish process, the most notable examples are Bushmasters made during the pre-Y2K build-up.  Bushmaster made more ARs during that period than all other manufacturers combined, and just didn't have time to correct this issue and still get rifles in the hands of anxious buyers.  There was a waiting list 6-9 months long during this period even with the huge increase in production.

Anyway, if you find the finish color objectionable, you can have it reanodized (I believe DPMS does this), or you can have a finish applied over the top, such as a teflon-based finish.  Or, if your rifle is brand new,  you can return it to Bushmaster for replacement.

-Troy