I've heard that during flowering plants must have ABSOLUTE darkness, but surely this can't be right if outdoor plants get moonlight and grow just fine.
That's my point. A plant in nature gets plenty of light from the moon and keeps right on flowering.
I found several articles of the internet that explain various theories about the effect of the moon on plants in general. Most talk about the gravitational force exerted upon the Earth at various phases of the moon and how they effect many aspects of the plants behavior. Some have mentioned the light from the moon as well. Here's a quote from an article by Marion Owen, co-author of the book "Chicken Soup for the Gardener's Soul". In it, she says:
"How is sowing, transplanting and harvesting linked to phases of the moon? One theory is that during the light (waxing) of the Moon, sap is thought to flow more strongly, filling plants with vitality and energy, favoring the planting and harvesting of crops that mature above ground."
Yes, it's a theory. But to think the moon has no effect seems impossible to me.