I'm guessing you have insufficient CO2/ferts for your lighting. What size is the tank? What light/photoperiod do you use? Dosing regimen?
I would advise against a blackout in this case. Trim the dead areas, lower the lighting, get your CO2 situation squared away,increase circulation, and dose, dose, dose. Aim for healthy plant growth-don't fight the algae. Fix the areas that need fixing and wait-I guarantee your tank will rebound.
Just enough light + ample CO2 + ample ferts = happy tank.