The short answer
Most Florida roofs should be soft washed every 1-2 years; tree cover, north-facing slopes or nearby water mean sooner.
What the black streaks are
A blue-green algae (Gloeocapsa magma) that feeds on the limestone filler in shingles, holds moisture and raises attic temps.
Never pressure wash a roof
High pressure blasts away protective granules and can void your warranty. Soft washing is the manufacturer-recommended method.
Signs it's time
Black streaks, green or white lichen, moss in valleys, or dark patches that keep spreading.