You've pretty much identified your own issues with the shot. The plusses - clear, clean and simple and good use of the ghetto lightbox (which is what, exactly?). Good even light - all the stuff you need for good product or object photography. Your pick for this particular challenge may not have been the best - as you note, most voters won't see black as a "light" color. But it does meet the challenge in that cameras use light - hence there's that tangential tie in! Given that the average voter spends about two seconds, though, most probably wouldn't have thought of that.
The crop is what is most problematic to me - too tight to the left side for no real reason in my mind.
I think you have a good handle on why you scored how you did, but more importantly it sounds like you had a good time setting it up, shooting it, processing it - and probably learned more than a bit along the way.
thanks for all the comments! i agree, i don't know what i was thinking with the border, and you're right, black isn't a light color, but i didn't have a lot to work with. I would've liked to have kept the whole strap in the frame, and actually, i'm not really happy with the crop, but i was having some issues in post processing and i got frustrated.