We present the first comprehensive theory for the phase behavior of thin polymer blend films. Based on the Landau-Ginzburg free energy functional, our mean field analysis incorporates the influence of finite size effects and surface interactions, and explicitly considers surface segregation. The procedure for calculating the full phase diagram is provided. In symmetric blends with neutral surfaces, the reduced critical temperature shifts t are obtained in exact analytical forms. Our predictions are in good agreement with our simulations. For polymers with N>100 (N being the polymerization index) in films much thinner than fully extended chain dimensions N1, a unique scaling behavior t is-proportional-to L-1 (i.e., the inverse film critical temperature depends linearly on 1/L) is found. When L much greater than N1 an Ising-type behavior to is-proportional-to N0.59L-1.59 is expected.