BAD, BAD, BAD Idea!!!
Very BAD idea!
On behalf of Adobe Systems Incorporated ...
Adobe Illustrator is not, repeat not, repeat yet again absolutely not a general purpose PDF file editor. The only PDF files that can be safely and fully edited in Adobe Illustrator are PDF files created by Adobe Illustrator via the save as PDF function and for which the retain editability option is specified. In such files, private data is added to allow such safe editing. You also must have all the fonts used in the PDF also installed on your system - Illustrator does not use the fonts embedded in the PDF (if any). For all other PDF files, editing in Adobe Illustrator may result in font changes, color changes, and content loss. Adobe Illustrator only supports a subset of the PDF file format and doesn't support mixed color spaces.
In the example given, the rich black likely came from a PDF file generated from an RGB application and fixing it in Illustrator may yield even more problems!
- Dov