If I export an interactive PDF with a form text field and other non interactive page items, Acrobat XI's Object Inspector tells me the page items are ICC sRGB but returns no info for the form field—anyone can confim this with Acrobat's Output Preview panel.
If I fill in the form field in Acrobat, save, and place the interactive PDF back into ID, and look at the page items in Separation Preview the values appear to be a conversion from sRGB to document CMYK. Text that was 100% black in the original is now 4-color, which is what you would expect of a black RGB object.
However the text in the form field is not 4-color, which means it could not have been an RGB object in the placed PDF. If I export the new ID document as PDF/X-1a, Acrobat now tells me the form text is Device-N Black
Here's a capture of the ID file with the interactive PDF placed with Black on and off:
You can see the form text is black only:
