As much as this advice is correct for painting, it's incorrect for design. What's wrong with black text? Nothing. Ironically, the site you link as a very thin font with very reduced contrast, so it's barely legible on my screen. The site is less legible than dark topfive and giving advice on design...
Form follows function. Barely legible text is very bad design.
> is correct for painting, it's incorrect for design
I disagree. It's a fundamental concept of Color Theory, which is a fundamental concept of design.
> What's wrong with black text?
Nothing. But you shouldn't be using absolute black. I tend favor #24242A.
I think it's unfair to call the site I linked to barely legible. The font is perhaps a touch too thin, but it's still very legible. Of course this is all subjective.
Form follows function. Barely legible text is very bad design.