I used it in the past on my old VW Rabbit GTI's black fender flares. You are correct it dies the part - kind of like liquid shoe polish. And like shoe polish, it wears off after awhile and you have to reapply it since the surface isn't really porous. I think I did it 2 or 3x a year on that car. It is an ok product, but when applying it can streak and can be hard to get a consistent finish and I think it can look worse when it wears off than it would have if you had never used it. Of course faded fender flares looked the worst!
I would try it in an inconspicuous area first and try it out for awhile before treating the entire bumper.