theres no point to growing in an outdoor greenhouse if ur blacking everything? that would kinda be indoor growing. ur headed towards a mess i think. mixing natures sun cycle with ur own artificial sunlight cycle i think could also just lead to a mess. i have a little idea though if im picturing ur situation right. ur just blacking out the walls right? so im assuming ur ceiling (which is clear because its a greenhouse) is going to be left alone for the sun to shine in. so here it goes: cover ur walls with whatever u want so it u cant see from the outside, but on the inside, line the walls with mylar so the sunlight that ur getting from above is reflected everywhere.

-this is just an idea, no idea if its better. to be honest if i were u id scratch the whole greenhosue idea alltogether since u cant utilize it fully and make a little closet garden in your home or grow in a nearby wooded area or even in your backyard. make sure u research for mold-resistant strains so u dont get bud-rot late in flowering if your growing outside.