False Conclusion: Don't use black on a web page because nothing in nature is actually black.
False Assumption: The black on a web page is actually pure black.
True Conclusion: Use black if you want to - it won't be pure black, it will simply be the darkest you can get.
PS. This is also true for paint: Black paint is also not pure black.
The thing about screens is that they are very bright, unnaturally bright compared to other things you see daily, because they are light emitting instead of reflecting. (They are very purposefully brighter than your surroundings so that you can still read them even if the room you are in is bright.) That means that when you put a bright white next to a dark black on the screen you're going to get a much higher contrast ratio than the same values in print. Yes, technically you could achieve the same values in print if you wanted to, but the fact is people interact with those higher ratios much less frequently.
So, yes, if you really want, you can make this your conclusion:
> Use black if you want to - it won't be pure black, it will simply be the darkest you can get.
But that's a pretty useless conclusion for anyone who wants to learn about what's going on. Instead, realize that in most cases the pure black you're using in an interface will feel unnaturally dominant, and will actually do more harm than good if you don't purposefully mean for it to be pure black.
If it was it would burn a hole in the canvas ;)