The Tefillin Bstim (boxes) part IVMBY 32:40 The following are some features of the ‘batim’ of the tefillin, which are halachically desirable, but not crucial:
1) Blackness of the ‘batim’: While we are always careful to paint the ‘batim’ black, and to touch them up when the color fades or is rubbed off, the actual ruling on this point is that it is not absolutely crucial to the ‘kashrus’ (i.e. validity) of the tefillin. Indeed, the blackness of the ‘retzuos’ (straps) is crucial, and there are authorities who rule that the same goes for the ‘batim’. However the official ruling of the Mishnah Berura is that while we should attempt to satisfy all opinions, if some or all of the ‘bayis’ was not blackened, it would still be kosher.
2) The blackness should be just liquid paint coloring the tefillin; not a separate layer or thick coating. Once again, this is a desired feature, but not to the point that the tefillin would be invalidated if it was not there.
3) The spaces between the four compartments should come all the way down. In a previous lesson, we introduced the concept of the four compartments of the ‘bayis shel rosh’, forged into one. We learned that the “lines” on the cubed top of the ‘bayis’ are actually spaces between the compartments. In fact, the compartments do not have to be split the entire way down. However, it is desirable that they do.