Black streaking on a Altamont 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.
Roofs below the escarpment stay wet longer than roofs a few miles out on the flats, and the tree cover through the village center keeps north slopes from ever fully drying between rains.
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.