cyano is a bacteria that is photosynthetic. its most limiting factors are phosphorus, iron, molybdenum, in that order. it is capable of nitrification and denitrification, so it will grow in the presence of ammonia, nitrate, and nitrite, but can also survive with only dissolved atmospheric n2. it is only capable of fixing nitrogen at about 10% of its maximum capacity during periods darkness, so although a blackout usually will not kill it by itself, it usually will stop it from spreading.
black outs work best at night because the stratification that occurs between different saturation levels of different gasses. at the bottom where most cyano is, the oxygen gets used up while the cyano uses it, while there is more oxygen at the surface, where dissolved gasses can exchange with atmospheric air.
removing phosphorus will slow it down dramatically, since it needs phosphorus to produce the lining of its cells. also, if you can find anything that removes molybdenum, it will prevent it from spreading as fast. molybdenum is used in special cells called fix nitrogen, which also happen to be the fastest reproducing cell. they also have a thick encapsulating layer around them, which helps to prevent oxidation through the cell membrane.
removing iron works just like it would with any plant. no iron, no chlorophyll. i dont think i need to explain that one.
if you want more information on cyano, look into getting Wetzels book, "Limnology, Lake and River Ecosystems" third edition. chapter 15 deals with nothing but algae and cyanobacteria.