Wikipedia provides a very useful explanation of rich black (or it did when I wrote this – you know that anyone can edit Wikipedia entries so it might say just about anything now!)
Just in case it has been hijacked by some troubled individual, we’ll provide a very short summary:
Rich black is a mixture of black plus one or more other inks to ensure that where black is used in the design we get a proper nice dark (rich!) black.
Black, black – so to speak.
If we didn’t, we’d get wishy washy black, grey black, not quite black, disappointing black etc.
Conversely a designer needs to know when not to use rich black – we can’t just go round using rich black everywhere. That would be wrong.
Incidentally, we don’t expect our clients to be interested in this, just to draw comfort from the fact that we do, and to take heed that many a new-designer-kid-on-the-block doesn’t.
You have been informed!