I have been buying grips for both old and new model Rugers. I am not sure about the blackhawk, but something was changed on the Vaquero some time around 2005. Two of my Vaqueros were made in the 90s and one made very recently do not take the same grips. The number on the frame will have a RED by it, indicating it is the newer one.
I think I participated in a discussion here about that maybe a year ago.