The Talmud (Shabbat 28b) teaches that the only materials qualified for “m’lekhet shamayim”, “the labor of heaven”, specifically, tefillin, are those from a kosher animal, or “from the permitted to your mouth”. The Ran (Rosh HaShanah, 6a b’dapei haRif) indicates, tentatatively, that this rule is applicable only to tefillin and to shofar, which fills a role comparable to “m’lekhet shamayim”. The Kozhaglover Rav (Resp. Eretz Tzvi, II, pp. 181-182) understands the category to be inclusive of all elements of kedushah, extending to shofar, which in its role as “remembrance” has a sanctified status.

Building on this connection, he innovatively suggests that the notion that Birkhot HaTorah are biblically obligatory (Berakhot 21a), can be interpreted expansively, indicating that all berakhot on aspects of kedushah are biblically mandated. He uses this notion to explain the implication that the berakhah on separating ma’aser is a biblical obligation (see Rashi, Devarim 26, 13) and the view that kiddush on Yom Tov is biblical (see Ritva, Sh’vuot 13). Similarly, the same would thus apply to the berakhot on tefillin; this could explain the practice of reciting the [questionable] second berakhah together with the corrective “borukh shem k’vod…”, rather than just omitting it as would be indicated by the rule of “safek berakhot l’hakel” (as questioned by Machatzit HaShekel, O.C. 25:10)

Elsewhere in his writings (ibid,. pp. 150-151), the Koghaglover discusses the question of whether the above rule would regulate the type of dye used on the outside of the boxes of the tefillin, a topic discusses at length in the Resp. Nodeh B’Yehudah (I, O.C., 1). The Koghaglover’s inclination, against other poskim, is to be lenient on this question, taking the assumption that the “blackness” of the tefillin boxes is an adornment and not integral to the mitzvah .