Rorie, what filters are they and what outputs are they. As in, are they spray bars, lily pipes, etc. I found blackouts didn't work, not because I didn't do them correctly, but because I didn't fix the root cause so it just reappeared. What I found was that it wasn't level of flow, as the amount of flow was sufficient, all my plants were waving around like they were in a gale. It was the efficiency of the circulation: there were dead areas, or areas getting less circulation, and this was where the bga was growing. The spray bar along the whole of the back of the tank noticeably fixed the problem: now the whole of the front of the tank has equal flow.
It's not just low nitrate that causes it, which means that just raising the nitrate level won't stop it from growing. It has multiple causes so needs multiple solutions.