Fifty Years after its premiere, George Gershwin's ''Porgy and Bess'' -- a love story set in a black ghetto in Charleston, S.C. -- appears to ahve passed a milestone: no longer is it likely to be referred to as ''Gershwin's, uh, opera.'' The hesitation about whether it's really opera was probably removed for good this week when ''Porgy'' appeared for the first time on the stage of America's greatest opera company, the Metropolitan in New York. It drew a standing-room-only audience and is to have 15 more performances this season.

The lavish grand- opera treatment was long overdue, a delay that may have been caused as much as anything by the widespread belief that something you like so much and whose words you can understand can't be opera. True, it's also a good stage show, but nobody can sing like a trained opera singer, and the music in ''Porgy'' merits performance by the sort of cast the Met could assemble.

In this case, it's an all-black cast, as Ira Gershwin, George Gershwin's brother, wanted for ''Porgy.'' Still, it isn't legally required for productions of the opera, and bass Simon Este,s who is singing Porgy at the Met, thinks it would be nice to break away from that particular convention in other productions of it.

''Even if Gershwin said it himself, I think it's an injustice to the work to put this tremendous restriction on it,'' he told the Associated Press. ''It makes it more difficult for 'Porgy' to be done. In Europe you can't find an all-black opera chorus. We have fine makeup today and coaches to teach languages and dialects. If this great lady Grace Bumbry, who sings Bess and I can sing Italian, German and French, obviously there are people who can teach singers to say 'gimme' instead of 'give me.' ''

If Gershwin never envisioned any white faces inhabiting Charleston's Catfish Row, Richard Wagner probably never envisioned Simon Estes in one of those sword-and-shield roles at Bayreuth either, but it's a good thing for opera-goers (and for Wagner) that he's been able to perform in them. For blacks, whites, Orientals, a suspension of race-consciousness and an end to typecasting is n order.

''We're beyond that,'' said Mr. Estes. ''Opera audiences want first of all to hear beautiful music and singing.''