Printing black in CMYK can be tricky. Even though it’s right there in the name (humbly represented by ‘K’), black isn't always as simple as it might seem.
CMYK works by fluctuating the values of the four colours of ink that are used – cyan, magenta, yellow, and black – that it lays down on the paper. If we were to define 'black' as any combination of these four inks where the printer uses as much black as it can (100%), we're still left with well over a million hypothetical and equally valid 'blacks' that you could print.
We’re often asked why black can look different, even on the same page. The fact is there’s no one definition of what ‘black’ means, and an RGB black converted to CMYK is going to look quite different to one that started its life in CMYK.
What to do about all this
1. If you're printing black text and the text is smaller than about 20pt, it should be 100% black, and not use any cyan, magenta, or yellow.
2. Don't use registration black. Ever. This uses more ink than digital printers can handle. At best, your text will be blurry, and at worst, it'll flake off the page.
3. If you're printing photographic work, feel free to leave images as RGB and let us or the printer convert the blacks.
4. If you're printing more graphical work and want full control over your colours (including black), edit the CMYK values individually rather than leaving them to chance. (Pantones and similar will be converted to CMYK by the printer, which usually does a pretty good job.)
5. Most importantly: be consistent. If one set of black text is defined in one way, and another lot in a different way, the colours will look different.