Older Ruger revolvers, such as this "flat top" .44 mag Blackhawk from 1957, were rife with plum parts. Check the loading gate on this example. I've been told that the plum color had something to do with the composition of the steel rather than the bluing process. At any rate, the plum colored parts are indeed desirable on these guns as proof of era authenticity if nothing else.
By the way, the Sambar stag grips shown are are original. Both stag and ivory were extra-cost options at that time.
John