The Kuumba Singers of Harvard and Radcliffe have become a prominent fixture in the local musical world during the past 20 years. Established in 1970 to give Black students at the Ivy League schools an opportunity to celebrate their culture, the Kuumba Singers offer a unique form of African-American spirituals, jazz, master choral works, poetry, and African chants. Robert Winfrey, musical director of the group since 1973, says he takes pride in the Singers for their music and, beyond that, for their skill as organizers who negotiate their own engagement contracts and plan nationwide tours.