I have been known to purchase the odd tube of ivory black but it's never right.
What this teacher was hinting at, but never actually said, was that there are several ways of mixing colours together which will produce a black.
The theory goes like this : if you mix equal quantities of the primary colours together you will get black. The variable in this is that you never get pure red, blue or yellow in a tube.
But there are colours that mix together that create a very passable black, and possibly more interesting than black straight from the tube, because a smidgeon either way gives a unique take on this non colour.
Here are a few combinations that I've come across: