Light is a factor, but so is organics, and cyanobacteria cannot exist without both. Not everyone has success with the dark tank method, because the organics are still there. I got rid of this twice in the same tank by doing serious substrate cleaning during the water change, reducing organics, and removing as much of the cyano at each W/C as I could. Try loosening it with your fingers, it will sink and be easily vacuumed out. I did not reduce lighting at all.