Here's an article about Ruger grip frames:
http://www.gunblast.com/Hamm_Ruger-SA-GripFrames.htm
In short, a lot of the earlier Blackhawks had aluminum grip frames. In addition, I believe they had aluminum ejector housings. I don't know what, if anything, was aluminum.
The aluminum parts were anodized black, so they weren't obvious. I've read rumors of them breaking if dropped on a hard surface, but I've never seen anything like that, or heard anyone who was first-hand say it had happened to them.
I believe the stainless models always used steel grip frames. It looks like Ruger's gone to steel on most if not all the blued ones by now.