It is not uncommon for roof shingles to turn black over time.
Roof mildew staining.
Spray the black stains first then the entire roof so the stains get a second dose.
When that happens it is generally caused by mildew algae mold or dirt stains.
You could replace all the roofing with new shingles dark enough to disguise the staining or with shingles laced with copper granules which are lethal to algae.
Wet forget moss mold mildew and algae stain remover hose end 48 oz 2 pack 3 9 out of 5 stars 120 rmr 86 instant mold stain mildew stain remover 2 pack 32 oz.
Roof shingles are exposed to a lot of wear and tear as extreme heat sun wind and precipitation come into contact with them.
The bleach degrades into salt which kills microbial growths and the sodium silicate does the same.
The less expensive solution is to spray wash the roof with a 50 percent mix of water and bleach to get rid of the algae.
Mix the soap and water well first then add the bleach.
This will cover 200 sq.