Do the maavarta cuts in Tefillin have to be blackened? Is this just a chumra?
The maavarta cuts that are seen from the outside must be painted black, as part of the bayis. See Mishnah Berurah 32:165.
However, the entire painting of the batim black involves a dispute among rishonim (the principle halachah le-Moshe mi-Sinai refers to the straps), some maintaining that this has the same status as the straps, and others writing that it is only a matter of Zeh Keili ve-Anveihu.
The Be’er Heiter cites from the Bach that the black of the tefillin themselves is not me’akev, and this appears to be the principle opinion, though the Be’er Heitev writes that one should not be lenient in this.
Furthermore, the rishonim cited by the Beis Yosef refers to the “ketzitzah,” which usually refers to the top part of the box alone. Although the Mishnah Berurah rules that the titurah is included, it is possible that here there would be broader agreement that it is not le-ikuva.
Thus, there is certainly room to understand that one is yotzei bedieved, yet one should be careful to get it painted.