Black Streaks are actually caused by black algae. Often confused as "soot" or "jet exhaust". This algae feeds on the limestone in the roofing shingle. Its "Bleeding" look is very unattractive and causes damage to your roof.
The Black Algae is caused by shading of trees, lack of Ultra-Violet rays from the sun to kill it off in its early stages, also like other alga's spores in the air. Usually present on the north facing roofs, but can appear anywhere. These unattractive streaks are an eyesore to your home. And also weaken the granular on your shingles.