Currently, MacStitch and WinStitch generate PDF files for emailing, and onscreen viewing, with colors described in RGB terms. 
Some professional printers translate those into CMYK colors before going to press.
In the process, in order to get deeper looking blacks for photos, they print black as ‘black plus some extra cyan,magenta, and blue’. Overprinting, in effect.
This is known as ‘Rich Black’.  But it’s not great for text and thin lines, especially if the printing process is not as accurate as it could be.. lines and text can smudge.
So we are now looking into ways to tell the Printers not to do that for lines and text.
In the meantime, if this kind of thing causes a problem for your work, there is a fix you can apply yourselves, instructions are below:
Using Acrobat Pro - Choose ’Convert colors’ from the ’Print Production’ tool bar on the right hand side. 
Then in the convert colors dialogue box, select ’Preserve Black’ in the ‘Convert options’ section near to the bottom. 
Change the output intent if required , then click OK. 
This will convert all colors which are ‘rich black’ to 100k  (full black, no cyan/magenta/blue)