The black streaks running down roofs are actually a hardy algae called Gloeocapsa magma. As the blue-green algae accumulate, they develop a dark, hardened outer coating, which results in the black stains you see. The algae feed on the limestone in shingles.
The algae will worsen and become more noticeable each year, trapping moisture and causing premature shingle aging and granule loss. If you don't like the streaks, reshingle with algae-resistant shingles. If the streaks don't bother you or you're not ready to invest in a new roof, install zinc or copper strips (available at roofing centers) along the top course of shingles. When rain hits the strips, it produces a solution that runs down the roof and keeps algae, moss and fungus from growing. However, the strips won't eliminate the existing algae.