Is There any Advantage to Retzuos that are Painted Black on Both Sides?

Should I buy Tefillin Straps that are Painted Black on Both Sides?

Traditionally, only one side of retuzos (tefillin straps), the outer side, is painted black. However, many sofrim (scribes) and Judaica stores have lately begun selling retuzos painted black on both sides, apparently based on a minority view of the Rishonim that they are more beautiful that way, and thus constitute a hiddur mitzvah (beautification of a mitzvah). This view may have a basis in kabbalah, as well.

However, it seems untoward to adopt a practice which follows a view that wasn’t practiced in the past. Indeed, poskim clearly state that the prevailing custom isn’t in accordance with this view. On the other hand, if one doesn’t do it on principle but rather for practical considerations, since retzuos that are painted on both sides are immersed in paint, and as a result the paint adheres better, it shouldn’t be a problem. #482

https://halacha2go.com?number=482

Practical Halacha: One minute a day. By Horav Yosef Yeshaya Braun, shlita, Mara D'asra and member of the Badatz of Crown Heights.