We present phase diagrams of binary blends (or mixtures) of a homopolymer H, with a block copolymer, A-B, in which there is an exothermic interaction between two types of segments, H and A. In trying to construct phase diagrams, we extend the approach of predicting phase separation behaviour in such binary blends where H is chemically identical with one of the blocks of the copolymer, e.g. A. A binary blend of a homopolymer with a block copolymer is divided into two distinct states, i.e, an ordered state and a disordered state. On the assumption that the block copolymer in the disordered state acts as a random copolymer, the free-energy change in the disordered state can be estimated by a model including the localized solubilization of added homopolymers, which is a modification of the confined-chain model originated by Meier. To determine coexisting phases under a given temperature, we examine the relative stabilities (i.e. the chemical potential of each component in the blend) of the two states. As a result, the concentration-temperature phase diagrams of blends of H and A-B can be obtained. From the predicted phase diagrams, it is found that the exothermic interaction of this system greatly increases the solubility of H into the A domains and affects the shape of the phase diagram.