I can't see the pic at my current location for some reason, but I think plum bluing can be a nice looking feature. I've got a P-64 with a plum slide (almost a brownish-purple) that I really like.
There are a number of reasons that chemical hot bluing can result in plum or other "off" colors. First, the chemistry of the bluing process: temperature of the salt bath, time in the bath, too high a concentration of salts (water evaporates quickly in the essentially boiling salt solution), or depletion of certain chemical solutes in the bluing salts. Second, the metal composition of the steel, since steel not only contains iron and carbon but many other metals such as cobalt, or manganese, or nickel, or molybdenum, or chrome at varying concentrations, depending on the alloy. I don't recall what different metal constituents lead to different colors other than the deep blue/black, but I'm just aware that the alloy composition does affect the color.