I think he took the wrong message away from his art teacher as I was taught the same thing.
But it was in regards to representing realistic things with paints. As he says shadows usually aren't truly black and painting with a dark blue as your darkener is usually a better idea.
You can use black but when you use it in paintings to darken colors or for your regular darks it regularly stands out to the viewer, black is supposed to be used in very limited quantities in artistic works. (Hell half the time people recommend not buying black paint when starting out but to mix other colors to create a black looking color[1])
This doesn't really switch over to apps and other things as they don't represent any realism but instead try to allow us to focus on specific things which black can be useful.
His example image with a big black square dominating the others is a terrible example and would be better if the bottom was a representation of the layout with black as the darkest color instead of the lighter greys and grey-blues. And what we would see is more contrast but little change in distraction.
But it was in regards to representing realistic things with paints. As he says shadows usually aren't truly black and painting with a dark blue as your darkener is usually a better idea.
You can use black but when you use it in paintings to darken colors or for your regular darks it regularly stands out to the viewer, black is supposed to be used in very limited quantities in artistic works. (Hell half the time people recommend not buying black paint when starting out but to mix other colors to create a black looking color[1])
This doesn't really switch over to apps and other things as they don't represent any realism but instead try to allow us to focus on specific things which black can be useful.
His example image with a big black square dominating the others is a terrible example and would be better if the bottom was a representation of the layout with black as the darkest color instead of the lighter greys and grey-blues. And what we would see is more contrast but little change in distraction.
[1]: http://painting.about.com/od/colourtheory/a/JimMeadersBlack....