On the link you posted previously, there is no data on the light intensity so nothing to compare it to. Watts is not very reliable as this is simply the amount of energy a tube requires to produce the light. So it is not easy to determine the intensity of that light. Knowing the tank dimensions however, which is a basic (or "high") 20 gallon, I would wonder if this is bright light. If it is, then reducing the duration may have minimal effect. There is a balance between intensity and duration, and one does not make up for the other; i.e., if the light is intense, reducing the duration will not help much, and similarly if the light is too weak, lengthening the duration will not help the plants get more light.
Second observation is that a week is not much time to gage any changes. Whenever I make changes to lighting or fertilization, I always allow three weeks minimum to see the results, sometimes longer. And here, what you are looking for is not for the existing algae to disappear because it will not (unless you manually remove it), but you want to see no increase in what is already there. Once you see this, you will know you have likely found the factor.
I still wouldn't recommend a blackout. If the initial cause is not corrected, the algae will only return. In my 20+ years I have battled brush algae several times, and I have always ended its increase by adjusting the light or the fertilizers.
Byron.