WHEN it comes to blackface art, the palm of accomplishment goes not, as you might suggest in the first three guesses, to Al Jolson, but to little Rosetta Duncan, co-star with her sister, Vivian, in "Topsy and Eva," at the Colonial. Not that Rosetta is a better comedian than Al. Maybe she is, and then again, maybe she isn't. But when it comes to applying the liquid burnt cork, she outdoes Jolson in every known direction.

  In her first appearance she is practically immersed in color, only a narrow strip amidships, as it were, being covered by a burlap garment, is minus makeup. Rosetta has gone the limit in makeup for this part. She gets to her dressing room an hour before the performance and she almost bathes in the ebony fluid.

  Her method of ridding herself of her acquired tint, is also interesting. It comes off gradually, as the show proceeds. In the second act, baggy trousers are part of her costume, and so some of the cork is removed. Later stockings and gloves are added, and more makeup departs, until finally only her piquant face retains its stove polish glow at the final curtain. Which method, according to Rosetta, is an immense improvement over removing the entire makeup after each and every performance.