Or maybe it's because black backgrounds are easier on the eyes, since you're not staring into a giant light source.
Only in a dark room.
It doesn't matter if the room is dark, or not. The screen is still a light source (barring various e-ink technologies).
Dark backgrounds with light coloured text are an enormous strain on the eyes in daylight or normal lit-room ambient lighting. Most dark background websites or blogs are a complete usability disaster.
You'll forgive me, of course, if I don't take your word for gospel on this. Of course, your opinion is shared by many, but the opposite opinion is also held by many, too. Don't get me wrong, I've seen my share of dark-background websites/blogs that are usability disasters, but the useability issues generally neither start, nor end there. If it's anecdotes you want, then if I had a dime for every light-background/dark-foreground website I've seen with usability issues, I could retire in comfort.
Black text on white or light-coloured background is by far the easiest in the eyes.
Maybe, maybe not. I'll agree that's true for printed dead-tree material (hey, there are even studies backing it up), but it's certainly not something you can accept as fact where computer monitors are concerned.
For example, I'm currently typing this message in a black-text on white background vim window, and I find myself frequently looking away to other places, because it's too damn bright (and no, I
don't have the monitor brightness cranked). Yet I can stare at a printed sheet of white paper for hours without the same problem. I also don't have the same issues when working in my default light-on-dark colour scheme.
Try reading an all-black background site for 10 to 15 minutes and then go walk around your normally lit house for a while. Notice anything disorienting?
No. I spend 8 hours a day in the shell and vim, with a light-on-dark colour scheme. I've never noticed anything disorienting when getting up to walk around. In my previous jobs, I worked in offices where the only lighting was via 40-60 watt desk lamps. Overhead lights were off, and windows were covered in heavy fabric, to prevent screen glare. For the last year, I've been working in an office with all the overhead lights on. (It's a refreshing change, even if they are flourescent tubes. :)) In both cases, I find my light-on-dark colour scheme
far easier on the eyes, and less fatiguing, than the inverse.