Problem is, we see dark things and assume they are black things. When, in reality, it’s very hard to find something that is pure black. Roads aren’t black. Your office chair isn’t black. The sidebar in Sparrow isn’t black. Words on web pages aren’t black.
Great observations and advice. Pure %100 black isn't natural, unless you're in space. Everything has some hue, texture or reflection to it. Rarely is anything in the real world an absolute black.
If you're a designer, take this challenge, it'll stretch your thinking and allow you make better design decisions. There's a whole spectrum of grays to experiment with. However, as with most design rules, there's always a time and place to break them.