I was asked this question a while ago "concerning black color in CMYK printing". The questioner said: "I know that black process color doesn't give you 'real' black from the print, and you have to mix it with others to get darkest possible black. Can you tell me which combination gives the darkest possible black color (extra black) without any 'light' points?"
There's no easy answer to that...
The black you get via CMYK (100%K) is as black as you'll get from just using black ink in any other way (though genuine process black ink is somewhat more transparent than the more normal spot black), but it is true that you get a richer looking black from mixing in a little of the other colours.
The problem comes with small text -- if you have 6 or 8pt text printed in more than one of the CMYK colours and the registration isn't perfect, you'll get a blurred effect that will look far worse than any improvement you'll get from the richer colour.
The compromise for many is to use a small proportion of yellow (say 10%) which won't show up badly if you get a slight yellow edge to the text and to use a small percentage of the background colour.
Another way is to create a "rich black" with a small proportion of say Y and M and apply this only to larger text sizes. The most common combination seems to be 100% K with 20% Magenta to give a warmer black, and maybe 20% Y. Using Cyan instead of Magenta gives a cooler black.
It's also possible to use higher proportions of the other colours, but then you come up against the problem of the total amount of ink you are putting on the paper -- and this will depend on the paper and the press. It can be as low as around 250% for uncoated papers (taking 100% as the equivalent to printing one colour solid). Overprinted black on solid colour need to be watched especially for this danger.
So, it comes again, as with so many such problems, to needing the advice of an experienced press operator, who will have a good idea from experience of what works in specific circumstances.
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