Although it is purely guessing, I don't think that it is actually important for a dark start to be completely dark. You just don't want to trigger algae explosions during the turbulence of the first weeks in a tank. In my limited experience with dark starts, abundant ambient light was never enough to trigger any visible algae in my tanks.
And it seems possible to me that microscopical, non visible and non dominant algae are an important part of the microbe assemblage, and it is positive to have them competing for their fare share of space. The more variety, the better, is what is said.
This is completely different than what a blackout is, referring to the strategy to wipe out algae in running tanks. When trying to kill well established algae as soon as possible, so as to not hurt the plants for too long, I think it is important to go pitch black.
Oh, and I also believe and put in practice the suggestion of massively increasing aeration of the tank during the dark start, with strong currents and great surface agitation.
. Pieces of cardboard everywhere, and go for 5-6 weeks 
