I've found algal blooms are usually just when something goes out of balance in the system. It WILL clear up on its own, but some people I've told to wait came back after 3 months rather annoyed and said it finally cleared. (Hey, I never told them it would happen overnight!)
I had the same problem with my 77 gallon last summer after doing an enormous cleaning and nothing seemed to help.. Water changes just aggrivated it (influx of fresh trace minerals and massive export of this "balance" I keep referring to but can't scientifically define). Reduced photoperiod had no effect. I didn't try blackout simply because the way the tank is set up in the room it would be an absolute nightmare to cover, and the neighbouring reptile tank wouldn't be impressed with me.
I finally broke down and bought a flocculant. A bottle of Green Water Clarifier (Laguna/Hagen pond liquid - I think it's just a more concentrated and cheaper version of P Clear) and I was set. Green algae comes.. Throw some poly floss in the filter (I didn't even do that) and the correct dosage of flocculant and the algae will clump and get drawn into the filter. I ended up having to add it 3 times over a week and the algae was gone. I even did a hefty water change after that and no algae blooms.
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