Sometimes the way a Q'tion is ask it will have a bearing on how a answer is given.
This board is full of professionals that know what will take black streaks off gutters,they are more than one product that wiil do the job.
If they did answer again as to does anyboy know it want be no different in what you can find though the search button.
Give thought on how to ask for info....does anybody know.....is like walking in a bar and asking..does anyboby know where I can find beer.
This is a powerwashing board and of course somebody knows the answer to your Q'tion.
In your search for how to get black streaks out notice how some of the Q'tions are ask and see which ones get the most info.
Most important is learning how to deal with "some holes" in a professional matter as to not look like one yourself as in idiot.
Good luck and enjoy