I think part of the problem is old fashion greed. Aluminum is more expensive than steel so Blackhawks should be more. Steel shrinks less than aluminum so fiting should faster. The latest Blackhawk frames are spray painted not anodized. While prep work for spray painting is less than anodizing it is still more than bluing.
I still believe there is quite a bit more labor cost associated with the fitting of the steel grip frames to the cylinder frame than with the aluminum grip frame.
take a look at an aluminum grip frame gun. the ears aren't flush with the cylinder frame, there will be overhang on the sides. the way it's been explained to me is the grip frame comes from the mold, it's finished (prep'ed, anodized/painted or whatever)is attached, tested for function and then sent on it's way.
A steel grip frame falls from the mold, is fitted to a particular frame.......ears are leveled and contoured with the grip frame. sides are made flush with the cylinder frame. (You can usually see the last few digits of ser # stamped on steel grip frames. showing it was made to fit a certain gun, similar to the cylinder) then the grip frame is prep'ed for bluing, and blued, or the stainless i guess is polished or whatever, then comes back and gets put on the same gun it was fitted for.
i'd guess both grips frames go through some "semi-finish" step imediately after falling from the mold...........but in a nut shell, a bit more effort/time is made to fit a steel grip frame than is done with an aluminum grip frame.