Reviewed by GREG KINGDirector: Benedict AndrewsStars: Ben Mendelsohn, Rooney Mara, Riz Ahmed, Ruby Stokes, Tara Fitzgerald.Based upon the controversial Olivier-award winning play Blackbird written by Scottish playwright David Harrower, Una is a confronting film that deals with some difficult and troubling and taboo material.Fifteen years ago, a 13-year-old Una (played by newcomer Ruby Stokes) had …