- open up a RGB .pdf you want to convert
- File > Document Color Mode > CMYK converts your RGB pallete to CMYK
- Swatches pannel > Add Used Colors adds every color swatch your document uses

file swatches used in document are now present in your swatches panel

select the color swatch you want to convert to 100%K, be sure to set 'Global' checkbox active this makes the color change in the entire document

* color shifts occur
** from time to time swatches from inside of groups or compound paths aren't added properly, so make sure to ungroup everything before you add/modify swatches
*** your document may have a variety of rgb blacks, so it will output a variety of cmyk blacks during rgb>cmyk conversion, in this case you have to make sure every black is 100%K black. if you have several black swatches to convert to 100%K, name conflicts might occur, so you should assign a different name in each case*
**** you may have to double check that every black is 100%K, just hit Add Used Colors a couple times until zero swatches are added to your pallete. if you change newly added swatches to 100%K and they continue to add, this means some swatch is assigned to an element inside of group or compound path, which prevents you from globally affecting it