When it comes to printing then all colors in a PDF document are transformed to the native color space of the printing device. If, e.g. a text uses a black RGB color then it is transformed to an equivalent CMYK value which contains contributions from all four color channels. In particular in mass printing applications these "rich black" values are not wanted, however, and it is required to use "true black" colors which use the K channel only. This article gives some ideas how this transform can be achieved.
Read more about richt black and how to manage it in our PDF expert blog.