How long to blackout Cladophora?

Hi, I thought I had eradicated cladophora in all my tanks, but two spots popped up in two of my tanks. Previously i used 10x doses (5ml / gallon) of 2.5% glutaraldehyde (excel) to kill it off. It was seriously -bad- before and I'd repeat, but I added some plants to the tanks that are not tolerant of excel in those doses.

I'd like to try a blackout. Like, black trashbags and blankets. How long should I leave it on? The moss that got infected by it that I'm trying to salvage will die from the excel, so I'd like to try another method. If the moss dies with a blackout too... then such is life.