A thought prompted by a revisit to the music of my youth (as people who've accumulated some mileage are prone to do.)
My argument, such as it is: his Eldritch Alien Overlords altered him in such a way as to make him immortal and (according to Sandy Pearlman, Blue Oyster Cult's longtime manager and the main architect of their grand overarching Lovecraftian-conspiratorial Imaginos mythos) averse to daylight (1); his self-conferred epithet "Eternal Light" evidently references the lunar "light that never warms."
In "Astronomy", he mentions his "gravelly digs", which might conceivably allude to a grave/tomb.
There's precedent for science-fictional rationales for vampirism; the first that comes to mind is Vampirella's origin story--vampires originated on the planet Drakulon, endowed with rivers of blood, and came to Earth in search of prey when their native resource dried up. No, seriously
--although apparently now she's been retconned as the daughter of Lilith; the Wizard has risen from the grave to slay, in turn, the Scientist who staked him.
Vampirism is another of the Cult's recurrent lyrical themes, as evidenced in "Vampire Tattoo", "I Love the Night", and "After Dark" (although this obsession seems to be the Bouchard brothers'.)
(1) from a 1975 interview for NME: It’s New Year’s Eve and Desdinova walks into the Four Winds Bar (which was a real place – an actual joint on Atlantic Beach). He plays this game with two girls which has to be completed in the six hours from midnight to dawn ‘cause he can’t stand the light. It’s so sort of… corrosive. There’s a parallel with the rose which is similarly over-fulfilled, a symbol of over-ripeness and decadence. The dog is Susy’s familiar and the carrier of starry wisdom from the actual dogstar. Lovecraft had this term 'starry wisdom cult’ which was so apt I had to use it.