Yes — document-proof fountain-pen ink exists. Montblanc Blue-Black, today sold as Midnight Blue, is an iron-gall ink and, according to Montblanc Customer Service, has been tested by the Bundesanstalt für Materialforschung und -prüfung (BAM) for light-fastness, water-drop resistance, smudge resistance and resistance to falsification attempts and is approved for documents including cheques. The package insert also marks the ink as document-proof. Other options mentioned are Noodler's Black (Bulletproof) and inks from De Atramentis. It is important not to mix iron-gall inks with other inks (they will thicken) and to clean the pen regularly.