I agree with padredw. I used to clean my roof religeously 2 times a season. The black streaks were terrible and hard to get off. I read on another sight that someone quit cleaning thier roof and the streaks stopped. So, I tried it and it worked.
I don't get black streaks anymore. I do clean the edges of the roof that you can see but I do not clean the roof. Haven't in two years and I haven't had any black streaks in as long. I get up there periodically to check things out and although my roof isn't white anymore, It doesn't seem to be getting any dirtier looking either. Now I get up there at the beginning of the season and spray it off real good with water and a brush, and alot of dirt comes off but it is still dingie looking but I don't have to fight the black streaks anymore.