There are many sources of roof staining and they vary in significance, from cosmetic to harmful to the roof.
Black streaks on shingles caused by
algae or fungal growth used to be limited to warm, humid
climates, but now this can be seen on houses as far north
as Canada. Some experts attribute the spread to the
increased use of crushed limestone as a filler material in
Limestone is economical and makes a
durable shingle, but the calcium carbonate in the limestone
supports algae growth. In algae-resistant (AR) shingles,
zinc or copper granules are mixed in with the colored stone
topping. When the shingles get wet, the zinc or copper is
released, inhibiting algae growth.
Warranties for algae
resistance are usually for less than 10 years since the protection
ends when the mineral washes away. Some shingles
have longer lasting protection than others due to a
higher percentage of AR granules.
The chemistry of roof shingles, their granule coverings, and substrates is quite different from other organic substances
that are home to many molds. Further, many newer shingle products include chemicals to retard black algae growth that
may also retard mold growth. Specific mold genera/species like to grow on particular surfaces - it's their food, and while
some molds are more choosy than others (for example mildews grow only on living plants), you'll need to look carefully
at a roof and the conditions around it (such as trees, and areas of sun or shade) and perhaps even sample the black
debris to determine if it is mold and if so what is its species.
The roof cleaning methods to remove black algae will probably work well for black mold growth on a roof as well.