Black streaks from rubber

[Montemar]

Under any window on my Broom 33 there are black streaks that come from the black rubber around or in the window fromes.
I thought that if I could change it for white rubbere I could solve the problem though new rubber rather thab 28 year old stuff may also do it.
Where can I go to get such a thing.

 

[ST840]

Here

Rubber Door Seals | Rubber Extrusions | Window Rubbers | Boat and Caravan Seals | UK

 

[Montemar]

Thanks