Blackening Tefilin with Special Ink

Is special ink required to blacken spots on parts of the Tefilin (not actual Parshiyot area but the area where the Retzuot connect through the bottom) and on the Retzuot that have faded.
Thanks so much

The ink used must not be made from any animal derivative. As long as you can ascertain that the ink is fully plant-based or synthetic, it is not a problem.