Are there any requirements on what you blacken the stripes of tefillin with? Can you use shoe polish or a regular magic marker? Where could I look for an answer?
The ideal thing to use is the actual paint they sell. They come in bottles that resemble a white out bottle, they also sell a paint “marker”. If you can’t get those you may use shoe polish or permanent black markers, however, you must make sure that they are not made from non-kosher-variety animal fats and stuff like that. Markers generally are not, shoe polish as far as I know is not but you should make sure. You can look more into this topic in Shulchan Aruch Orach Chaim Siman 33:3 and the Mishnah Brurah there 33:19. See also the sefer “Keses Sofer” Siman 23 sif 2.