Here's a very good step by step when I had recently walked someone else through this matter via PM http://www.tropicalfishkeeping.com/f...lackout-35508/
What's your water exchange maintenance (How often & How much)? And yea go ahead and test them NO's & Ammonia. A blackout will help yes but if the root cause that made it grow in the first place like that doesn't get taken care of odds are it may come back.
How long are you running your lights (and what wattage do you have over what size tank)?
When I did mine; I did a about 50% water exchange. Blacked it out compleatly with blankets for 4 days; then did another large w/c and super vacuumed the heck out of my gravel and all these 'dust bunnies' that the Cyanobacteria turned into and its been gone since. Thou I have to add for a few weeks after that I had my lights 5hrs on 4 off 5 on.