출애굽기 13:22의 Responsa

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.