^^^ those are all too simplistic... anyone seriously into color psychology can tell you that.
White is stereotypically related to purity. But it can also be anonymity, neutrality, infinity, flatness, a-dimensional, timeless, openness, lightness (both weight and mood)...
Black is indeed a color related to power and intensity. It can also be mystery, forbidden, stylish, expensive, privateness, exclusivity, gravity,
gravitas, seriousness, stability, depth...
If you consider the forum as a real "space".. well the quality of openness, lightness, flatness, and purity work against it, in the sense that in an large "open" public place people behave quite robotically, in the sense that all is made lightly of mood and lightly of thought, flatness of ideas, and purity as in an oppressive or intolerant kind of cleanliness, a totalitarian cleanliness.
The dark has the intense
gravitas and privateness of a small group of friends having a profound late night discussion at home; not the cacophonous trivialities that are shouted at uniformly lit shopping malls and airports.
"In a small room one does not say what one would in a large room" - Louis Kahn
BTW there is no such thing as "regular green"
EDIT: More thoughts
Besides, black recedes while white moves forward. Thus the emphasis is brought on the text and the avatar, that seem to be the only things there.
On the white-dominant forums, the background itself robs a lot of the attention from the text, indeed the text seems to get lost amidst the white and blue boxes... it looks much more busy.
Black background with white font is a common strategy to emphasize text, used in magazines, ads, and cinematography... It's also one of the simplest ways to make a minimalistic powerpoint presentation that looks stylish and doesn't blind your audience (I use it all the time).
)
). 
