AFRICAN PRINTS* Tanzania’s governoent-owned Friendship Textile Mill* after a year in operation, is employing some 2,600 workers to turn out traditional African cotton prints called, in Swahili, khanga and vitenge. Khanga, indigenous to East Africa, originally was stamp-printed by hand and dyed with natural dyes. Vitenge is the brightly-colored African print worn commonly throughout the continent. Rich reds, oranges, yellows predominate in both. The Friendship Mill, one of about 40 companies owned by the Tanzanian National Development Corp., was begun last year to take advantage of the country's vast supply of high-quality cotton. New York's Ha2el Blackman, designer and owner of the Treehouse Boutiques in Manhattan, a pioneer 20 year3 ago in adapting African materials to Western fashions, visited the mill last Fall and brought back a selection of materials for her new collection and for use in redecorating her own home.