Black streaking on a East Greenbush roof is Gloeocapsa magma, a hardy algae that feeds on the limestone filler in asphalt shingles. It starts on the shaded slope and spreads across the whole roof over a few seasons.
The ridge neighborhoods above the Hudson catch weather from both directions, and the older asphalt roofs through Columbia Heights and Defreestville show algae streaking earlier than newer builds.
Pressure washing a roof strips granules and shortens its life. We use the low-pressure soft wash the shingle manufacturers actually recommend: apply, dwell, rinse gently. Streaks are gone the same day and stay gone for years.