In the Greek magical papyri (Papyri Graecae Magicae or PGM), lines 2307–2314 of papyrus IV were translated into English by Hans Dieter Betz as follows:
The hair of a virgin cow, the seed of Pan
Fire from a sunbeam, colt's foot, spindle tree
Boy love, bow drill, a gray-eyed woman's body
With legs outspread, a black sphinx's / pierced vagina
All these are symbols of my power
The bond of all necessity will be
Sundered, and Helios will hide your light
At noon, and Tethys will o'erflow the world
Hans Dieter Betz (1986). The Greek Magical Papyri in Translation, Including the Demotic Spells, p. 80. University of Chicago Press.
A digitized version of papyrus IV is available via the Bibliotheque nationale de France.
What is puzzling to me is that nearly all the contents of this passage are easily identifiable: Helios, Tethys, Pan (seed of Pan seems to be parlance for some kind of herb, if other PGM texts are of use here). Yet, what stands out as not being easily identifiable is the "black sphinx." I have not encountered it elsewhere in Greek or near-eastern mythology. Of course there is a sphinx in Oedipus, however it is does not take on any epithet that's similar to "black sphinx".
I've also tried googling around the term "black sphinx" but little-to-nothing comes up. This does not surprise me because it's a very specialist kind of field. Many of the texts that classicists study are not indexed well by search engines.
From my own musings, if it's anything at all, then either "black" was included for a reason, but the motivation is unknown to me. Or, it may be that "black sphinx" is an entity unto itself.
Assuming "black sphinx" was included with some kind of intent (not just a throwaway predicate), what, if anything, may we refer to elsewhere in the literary/mythological world of the ancient Greeks to help us make sense of the allusion to a "black sphinx."?
Not for the faint of heart: https://kosmossociety.chs.harvard.edu/wp-content/uploads/2019/04/CHS-Greek-Magical-Papyri-Examples.pdf