''The Masked Ball'' is Herbert von Karajan's last opera recording, the piece he was preparing for last summer's Salzburg Festival when he suffered his fatal heart attack. Back in the mid-19th century, the censors made the composer change the story from the assassination of King Gustav III of Sweden to that of a mythical ''governor'' of Boston, but in any setting this is one of Verdi's better efforts. Domingo, who seems hell-bent on recording every tenor opera role ever written, is in good voice as the doomed king -- you can come out now, Giuseppe, the censors are gone -- even if the rest of the cast is only serviceable. Karajan's leadership, though, is broadly paced and finely detailed: Verdi as if Wagner wrote the music. Typical of Karajan, but somehow it all works.