By observing that the plum colored triggers (and sometimes a few other small plum colored parts, such as extractors) aren't on most Lugers, I suppose it could be considered bad craftsmanship, but I don't suppose the parts looked bad at the time, or they'd have been refinished.
My guess is that age exposed the flaw, and as it's irrelevant to the function, it wasn't worried about at the time. It also could be that the plum color didn't appear until years (or decades) later, so they weren't "defective" during their original expected useful life. Maybe it took 30 years for them to go plum. I've no way of knowing. I have a few parts on my 98k's and Lugers that are plum, and some guns that have no plum at all. There are salt bluing methods, and rust bluing methods. Somehow, something was different in the process or the steel, or the parts would match.
If it was something such as storage grease that stained the steel, a perfectly plum trigger and extractor wouldn't be on a perfectly dark blue Luger. It had to be the finishing process that was different.