Responsa do Wyjścia 13:22

To my mind, it is hard to place the adjective ‘black’ on the tefillin straps, as straps were not mentioned at all. In addition, the text combined the terms ‘square’ and ‘black’; just as square describes the boxes, so so the term ‘black’. Then the Jerusalem Talmud gave a reason for what was stated in the Mishna, as to why round tefillin are potentially hazardous and do not fulfill the obligation (mitzva) of donning tefillin.