The phase behavior of binary blends of A homopolymer and symmetric AB diblock copolymer is studied within mean-held theory. The occurrence of lamellar, hexagonal, and body-centered-cubic phases is examined in the weak to intermediate segregation regime and over a wide range of homopolymer length relative to that of the diblock. This ratio is found to be a crucial parameter for the topology of the phase diagram. Several different classes of behavior are established, and comparisons are made with experiment.