The "plum" color is a result of interaction between the alloy of the part and the specific chemical make up of the bluing compound used.
It's not something done intentionally. Its often not something you can even be aware of, until afterwards, sometimes long afterwards. Batch #A234 might produce parts that turn "plum" with age and bluing batch #A235 does not. When the stars (in this case all the chemical interactions) line up in just the right way, it happens.
Sometimes parts come out of the bluing looking plum and get used, sometimes they look normally blued and turn plum over decades.
it doesn't hurt anything, unless it upsets your personal sense of what looks good.