Having kind of a “duh” moment.
Although I had connected Dionysos with the Black Sun well before I experienced the dream wherein he gave me his controversial spoked symbol it took me a while to realize that this title had been hiding in plain sight all along:
After the precinct of Zeus, there is a temple of Dionysos Nyktelios, a sanctuary built to Aphrodite Epistrophia and an oracle said to belong to Nyx. (Pausanias, Description of Greece 1.40.6)
Most scholars translate Νυκτελιος as “Nocturnal” or “of the Night” which makes sense for a number of reasons: Dionysian rituals (especially of the orgiastic variety) often take place in the dark; the close proximity of this temple in Megara to that of Nyx, Goddess of the Night; and the important role Nyx plays in Orphic cosmologies where she is daughter, wife and successor of Phanes, the primordial Dionysos.
However we get a better sense of what this title meant to his devotees from Plutarch’s On the E at Delphi:
As for his passage and distribution into waves and water, and earth, and stars, and nascent plants and animals, they hint at the actual change undergone as a rending and dismemberment, but name the God himself Dionysos or Zagreus or Nyktelios or Isodaites. Deaths too and vanishings do they construct, passages out of life and new births, all riddles and tales to match the changes mentioned. So they sing to Dionysos dithyrambic strains, charged with sufferings and a change wherein are wanderings and dismemberment.
Plutarch goes on to both equate and contrast Dionysos with Apollon in a manner similar to what we find in Eratosthenes and Macrobius, a tradition which hearkens back to primitive Thracian Orphic beliefs wherein Apollon is a manifestation of Helios above the Earth, while Dionysos is Helios in the Underworld, much as the Egyptian solar deity Rē fuses with Osiris in the Duat.
Nyktelios can certainly be parsed as “Nocturnal” but if you break it down a different way – Νυκτ- “Night-dark” and έλιος “the Sun” you get … Black Sun.