Ruger shipped a few Blackhawks with brass gripframes around the time yours was manufactured. Based on your post, I'm assuming you've already got a letter from Ruger identifying its "as shipped" configuration. If not, you should request one; many folks have installed aftermarket gripframes on their Blackhawks, but original "Brassies" are worth substantially more than their more common contemporaries.
My shooter grade .357 below was originally shipped with a brass gripframe. A former owner, alas, changed that.
You might want to post an inquiry on value in the revolver section of RugerForum.com. Its moderator ("Flatgate") is a very accomplished Ruger collector and seems to have a particular affinity for brass gripframes, both factory and aftermarket.