Tzipporah, the wife of Moses, is referred to as a Cushite. Why is she described as a Cushite when she actually was a Midianite?

The Cushites historically were a dark-skinned people. A dark-skinned woman is a rare visage in the Torah; thus, the Torah is metaphorically describing that Tziporah was unusual in her deeds. (Talmud, Moed Katan 16b)

Commentaries explain that just as a dark-skinned person is immediately noticeable, so too an individual described as a Cushite possesses a character that is obvious and clear to all. (Ben Yehoyada, ibid.)

What was so great about Tzipporah? She was exceptionally modest and unassuming in her actions. (Talmid Rabeinu Yechiel of Paris, ibid.)

Alternatively, she was capable of transforming the physical into spiritual and darkness into light. (Shaloh, Shavuot 242 and 247).