Founded in 1967, the Blair String Quartet rotated through two different first violinists during its first four years of existence. Then, in 1972, a young up-and-coming virtuoso named Christian Teal took over the first chair — and the rest, as they say, is history. This Saturday, after 42 years, Teal will give his final concert with the quartet. To mark such a historic and bittersweet occasion, the ensemble — violinists Teal and Cornelia Heard, violist John Kochanowski and cellist Felix Wang — will perform Beethoven’s most ambitious chamber work, the String Quartet No. 13 in B-flat major. (The musicians, by the way, will play that work’s original ending, the magnificent Grand Fugue.) Pianist Melvin Chen will also join the ensemble to perform Ernest Bloch’s Quintet No. 1 for Piano and Strings. The concert is free.