That is, in fact, the case. The font characteristics are specified in the PDF file. Here's what I did to make my filled-in text black.
WARNING! Do this on a copy of your PDF file so when you screw up you can make a new copy and try again!
Using a good text editor like BBedit which doesn't screw up binary characters in the text file, replace all occurrences of "0.502" with "0.000".
What this does is changes the blue text to black in objects like this:
20 0 obj
<< /Border [ 0 0 0 ] /Rect [ 36 689.998 236.85 703.999 ] /T (topmostSubform[0].Page1[0].f1_04[0])
/F 4 /BS 1929 0 R /Subtype /Widget /DA (/Helvetica 8 Tf 0.000 0.000 0.502 rg)
/MK 1930 0 R /AP 1931 0 R /DV (PETER E) /V (PETER E) /StructParent 4 /FT /Tx
/Type /Annot /Ff 8388608 >>
endobj
Which become:
20 0 obj
<< /Border [ 0 0 0 ] /Rect [ 36 689.998 236.85 703.999 ] /T (topmostSubform[0].Page1[0].f1_04[0])
/F 4 /BS 1929 0 R /Subtype /Widget /DA (/Helvetica 8 Tf 0.000 0.000 0.000 rg)
/MK 1930 0 R /AP 1931 0 R /DV (PETER E) /V (PETER E) /StructParent 4 /FT /Tx
/Type /Annot /Ff 8388608 >>
endobj
Note: "PETER" is the text I entered into the form that was blue and is now black. I tried changing the font to "Courier New" but that didn't have any effect. Probably because Courier New isn't defined within the PDF file.
You can probably do the same in the Terminal with awk, sed, ed or some other command line tool, but not nano. Nano screws up the line breaks and binary streams. Probably not vim, either?