Many erroneously assume that Blake-light tragedy is a reference William Blake, who was indeed an enormous influence on Ginsberg and whom Ginsberg does make allusions to elsewhere in other works.
However, it is actually a reference to an obscure incident which occurred in Denver, CO where Blake Street is found. Kerouac, Ginsberg, and Cassady spent time in Denver, which features numerous book stores, restaurants, murals, and even apartment buildings that all pay homage to the Beat Poets as a result. Ginsberg makes this more explicit in the following passage: "who journeyed to Denver, who died in Denver, who came back to Denver and waited in vain, who watched over Denver and brooded and loned in Denver and finally went away to find out the Time, and now Denver is lonesome for her heroes".
I work at the Denver Art Museum and five years ago met Ginsberg and Orlovsky's personal chauffeur during an exhibition on women artists in the abstract-expressionist movement who confirmed to me that Ginsberg was talking about this street as he had brought it up to him while being driven around the city in the '80s.