Get some pvc cleaner and clean the black marks of
Astonish paste works fine.
With regard to op. How many houses have you got the problem with?
We had a couple of complaints about the same on bottom sills because my son insists on scrubbing the bottom of the window from the side which means the edge of his brush runs up and down the sill.
However, no complaints on top windows as he can't do that.
We have also noticed that there are 'scratch' lines in some sills that originate from the extruding process when the sills are first made/formed, so were there before the windows were fitted.