Mysterious black stuff on redwood deck

Redwood deck in Western Wyoming 7200' elevation, north side with summer sun and winter shade and snow. They used Cabot when built 5 years ago and we reapplied same Cabot after 2 years. Last year most of the color was gone on horizontal surfaces and there was black residue coming off on hands and clothes wherever touched. We scrubbed it by hand with just water and didn't re-stain. This spring more black residue so we just cleaned it with a power washer. We don't want to let it go grey another summer but have no idea what to do-- we're desperate for advice! Thank you.