The black rubber seals on double glazing do simply break down over time and in my experience, we come aling,clean the windows and the black surface leaks into the white frame a little more, this is hapening anyway to them. It is a design fault. It doesn't happen to the white seals, mybe they are plastic (upvc) so the sun doesn't break them down so easy. I have found some complain, and that is my explanation.
Pj