Must the black side of the tefillin straps (retzuot) face outward, and what is the consequence if they become reversed?

Tzitzit & TefillinTefillinStraps (Retzuot)

Synopsis

The black side of the straps must face outward for both shel yad and shel rosh. The critical zone for this requirement is the portion that circles the head and arm; subsequent wrapping and hanging portions are less critical. If reversed, the pious practice is to fast or give charity.

Early Acharonim
Late Acharonim
Lenient
Strict
Lesser cited
Major authority

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