What is Rich Black?
Rich black is a term used to describe the process of overprinting normal black ink with additional colours to make the black appear darker. It works because a mix of Cyan Magenta and Yellow inks in equal proportions should look black without any black ink present, so in effect you are doubling the amount of black ink over that part of the page, making the area darker.
Here are the values we use on offset presses for rich black.
100K - 60C - 40M - 40Y
--from the Scribus Archives
And some further advice:
For text below 12 points, I would advise not to use rich black with 4 inks.
You could use a 100K and C50 instead, if you really want it. But again, registration issues might occur with smaller typesize. Depending on the font and face you use. You'll be a better judge. I only point to the issues that might or will occur and I only want to explain a bit so you understand what's going on. You could also discuss this particular issue with your printer.