Black Cat paperbacks originally had "A Black Cat Book” at the top right of the front of the wrappers with the series number listed on the spine. These releases from BA-1 in 1961 through BA-58 in 1963 corresponded to the following prices:
BA = $.50
BB = $.60
BC = $.75
BD = $.95
With BC-59, Henry Miller's Tropic of Capricorn, published August 5, 1963, but issued previously on July 16, 1963, as a Dell-distributed Grove paperback at $.95, Grove dropped the correlation between series number and price. Grove likely simply retained the $.95 price from the Dell edition and began giving each new Black Cat release from then a BC- prefix.
Beginning with the next batch of Black Cat paperbacks, issued summer 1964, Grove changed the Black Cat line to an sub-imprint of Evergreen, labeling new releases as Evergreen Black Cat Books. This new name is first mentioned in reference to Henry Miller's Black Spring, released as BC-61 at $.95 in June 1964. Brecht's Baal / A Man's a Man / The Elephant Calf comes earlier in series order, at BC-60, but was not published until October 1964.
Grove switched to B- series numbers in either late 1967 or early 1968 (see, e.g., Bulgakov's The Master and Margarita, issued c. November 1967, as B-147, and Tuli Kupferberg's 1001 Ways to Live Without Working, issued January 1968 as B-141). It appears that Grove dropped the appellation "An Evergreen Black Cat" in 1977, from which time books in the series returned to the label "A Black Cat Book."
https://www.jahsonic.com/GroveBibliography.html -- lists BC 1-566
Grove Press Starts Mass Market Line, PW, June 19, 1961, at 44.
Italicized note, PW, Mar. 9, 1964, at 65 (announcing that Henry Miller's Black Spring, due out April 15, 1964, and Bertolt Brecht's Early Plays will launch a "new series by Grove Press, the Evergreen-Black Cat Books, which combine Grove's two paperback lines. The series will be printed in a standard size (4-3/16 x 7-1/16 in.) and fit mass-market paperback racks. They will sell at a low price.")