> Are you 100% sure the thing you're making #000000 is the absolute darkest thing on your site/app you'll ever need to declare?
Assuming you have a well-managed CSS architecture, you could use black for the darkest thing on your site now and change it later if it absolutely becomes necessary! I don't quite follow the logic of not using pure black now because you might need it in the future, since that same argument will apply just as much at that future time! Besides, there are only 256 shades of neutral gray (black #000000 to white #FFFFFF), so even if your "black" background is #333333 one could still argue "Are you 100% sure there won't be more than 50 darker shades of grey on your site you'll ever need to declare?"
Assuming you have a well-managed CSS architecture, you could use black for the darkest thing on your site now and change it later if it absolutely becomes necessary! I don't quite follow the logic of not using pure black now because you might need it in the future, since that same argument will apply just as much at that future time! Besides, there are only 256 shades of neutral gray (black #000000 to white #FFFFFF), so even if your "black" background is #333333 one could still argue "Are you 100% sure there won't be more than 50 darker shades of grey on your site you'll ever need to declare?"