Black streaks where do they come from

Old Snowdrop

So my caravan had lots of black streaks down the side, I cleaned it and polished it, I cleaned all the window rubbers and 'oiled' them. Sill got black streaks, not necessarily under the windows so it was not the window rubbers. I have now been at the roof and cleaned that off. It was black, now is clean and shiny. This week we have had lots of rain and the black streaks are back. More prevalent on the top at the front and rear, but there is a large one under where the front gutter deposits the roof run off. Remember I have cleaned the roof.

Wiping this large one with my thumb removes it as if it was a powder.

so where do these black streaks come from?