You can sign USCIS forms in either blue ink or black ink it doesn’t matter which color you use. The words “type or print in black ink” or “type or print legibly in black ink” is referring ONLY to the answers section and not to the signature itself. I have never seen a petition rejected because of the color of the signature. In fact the official USCIS instructions regarding signatures doesn’t even mention pen color, which you can find here.
However, all signatures must be originals. If you submit a petition with a photocopy of a signature or the signature appears to be a photocopy you will receive a Request for Evidence. In order to avoid this situation press firmly when signing your I-129F petition and all other USCIS forms to engrave the signature on the paper and obviously never submit a photocopy.