Exporting to PDF with the press setting will convert your RGB images to CMYK but should not do any harm to black.
but that that will also convert text from Black to CMYK
Is your text defined as an RGB black swatch? If it is change the swatch definition to CMYK 0|0|0|100 black and export using a PDF/X preset with Output>Destination set to Document CMYK. In that case any document CMYK colors including CMYK blacks will export unchanged.
- It would be better to REPLACE the RGB swatch with the [Black] in brackets. This will overprint. A self defined black will not overprint. (At least with default settings.)
- You can convert on PDF export, use PDF/X-1a which allows only CMYK space.
- In your Color Management Settings in CMYK Policy select keep values for CMYK, this will avoid a CMYK to CMYK conversion which would split up K-black to 4c-black.
In your Color Management Settings in CMYK Policy select keep values for CMYK
The Color Setting's Preserve Numbers policy only affects placed CMYK files and does not protect native color or swatch numbers.
On export you can protect native color CMYK values by setting the Color Conversion to Convert to Destination (Preserve Numbers), but Convert to Destination with the Destination set as Document CMYK will also leave native CMYK unchanged.
In the end you can't control the output when you hand off the PDF—PDF/X-1a makes CMYK-to-CMYK conversions less likely but not impossible.