The quick answer, Algae or as it’s scientifically known, Gleocapsa Magma. The base of the shingle contains asphalt which includes limestone. The stains begin after the algae digests the limestone in it for food. These living organisms start out as small green or black spots then spread rapidly.
So, wherever you see those black roof stains, the asphalt shingles are getting damaged.
The stains will expand until it completely covers the roof in the shaded areas if it isn’t cleaned off.
The other culprit, Moss, does the same damage to your asphalt shingles. A seasonal roof cleaning service keeps the destructive algae and moss off the roof of your home.