Blackened surface of conglomerate stones

Some of the stones of the conglomerate rocks exposed to sun and weather have a blackening that I will guess is a growth of blue-green algae.  Rachel Carson in some of her writing mentions the black zone of the uppermost tidal regions of the seashore, visible in every part of the world, habitat of blue-green algae.  Here is a link to p. 31 of her book The Edge of the Sea describing this black zone.  I don't find very much writing available on epilithic algae that this photo might depict, probably it deserves more study.

Photo taken Feb. 23, 2015, along the trail near the location of the previous two posts.

Another link to cyanobacteria tidal zone: