While I have been cleaning 'trad', I have come across a lot of these seals where the black rubber seems to have perished/broken.
The black smearing happens when the squeegee, or scrim, touches the seal and then the frame, If the frame is wiped immediately, it will come off again and does not leave a permanent mark.
I think that with WFP the constant flow of water does not give the 'black stuff' time to permanenly mark the UPVC, and if rinsing is done properly any contaminated water should be washed away.
I think (and hope) that's the case, anyway.
Mike