Our practice is to recite the bracha of al mitzvas tefilin before donning the shel rosh, but to add barch shem kvod malchuso as if this was a bracha l’vatala. If we are concerned for Rashi’s opinion that the bracha is unnecessary, than based on the rule of safeik brachos l’hakeil shouldn’t we simply omit the bracha entirely?
The Aruch haShulchan creatively suggests that baruch shem is not said as a response to a potential bracha l’vatala, but is said simply as a statement of praise to Hashem’s name which is represented by the tefillin shel rosh – v’rau kol amei ha’aretz ki shem Hashem nikra alecha v’yaru m’meka. He further adds that the bracha of al miztvas tefillin itself is not a birchas hamitzva, but a birchas hashevach, as we never find two brachos on a single mitzvah act.
I do not quite understand the redefinition of this bracha as a birchas hashevach, as a birchas hashevach is normally recited only after the fact; here, the bracha is recited specifically over l’asiyasan, before the tefillin are donned. Perhaps one might explain the two brachos here relate to two distinct kiyumim of tefillin: 1) the ma’aseh hamitzvah, the act of donning the tefillin, which is covered by the bracha of l’haniach; 2) the kiyum of wearing tefillin, which is covered by the bracha of al mitzvas. This may explain why according to Rashi in the case of interruption a seperate bracha of al mitzvas is recited on the shel rosh and we do not simply repeat the bracha of l'haniach. Clearly hefsek is not a mechayeiv of shevach; Rashi must see the extra bracha as a new form of birchas hamitzvah.