Well this is what I went through the last I time I had BGA. Correct, it is only caused by a lack of NO3 or/and a lack of circulation at that particular spot. If you don't believe, try this, put a power head just above the place with BGA, the BGA will dissappear, assuming you have sufficient NO3 of course. The last time I had BGA, my NO3 readings were zero. I added KNO3 until my NO3 reached btw 5-10mg/l. After I added the KNO3, all BGA disappeared. No need blackouts. Blackouts will also work, but in the end you will still need to address the NO3 or circulation issue, if not the same problem will occur again.

I bought my KNO3 (Green Light Stump Remover) from MidValley's ACE Hardware and thats the only place in PJ/KL which I know of at the moment which sells this.