KENWOOD — Constellation Wines U.S. on Monday informed the 10 employees at the Blackstone winery and visitor’s center in Kenwood that it would be closed as of March 1 and possibly put up for sale, as production of Blackstone Sonoma Reserve wines has already relocated to Sonoma.“We determined it could be done without affecting quality,” said spokeswoman Nora Feeley. “It was important to keep it in Sonoma.”