Benjamin Millepied, the ‘‘Black Swan’’ choreographer who helped transform Natalie Portman into an obsessed, paranoid ballerina for the film and later married the actress, was named director of the Paris Opera Ballet on Thursday. Millepied, 35, is a former principal dancer with the New York City Ballet who left in 2011 to create a dance company in Los Angeles, L.A. Dance Project. He’ll start at the Paris company in October 2014. Portman and Millepied, who have a son, met during the making of ‘‘Black Swan,’’ Darren Aronofsky’s thriller that stars Portman as a ballet dancer.