I have a P7M8 from 1989 that looks new, no plum on the slide. I also have a P7 surplus gun from 1983 that shows a bit of holster wear from it previous BMI user, but again no plum color.
The finish is durable as any other blue finish. I don't know why some exhibit the plum color as they age, and some don't. The plum color does not bother me in the least, however.