I've heard and seen religious Jews painting the backs of the tefillin straps black, and they claim a kabbalistic reason somewhere in the Schulchan Auruch. Does anyone know anything about this, the reason, and source?
Welcome to Mi Yodeya Sam! Nice question.– Bruce JamesDec 24, 2013 at 0:12
There are opinions that this makes the Tefillin a more beautiful Mitzvah. Although the Arizal may be one of them, the Rambam, the Or Zarua and others who are not regarded as kabbalistic in their rulings also rule this way.
The general custom is to not do this, but as you noted, there are those who follow those opinions. Nobody puts it as a requirement (except perhaps some Kabbalists, depending on their understanding of the Arizal), but rather as a Hiddur Mitzvah.
1There are opinions that the sides of the straps are required to be black since they are visible externally while being worn, and generally the easiest if not only way to get straps like that nowadays is to get the ones painted on all sides.– Double AA ♦Oct 23, 2017 at 21:28