The effects of the addition of a small amount of C-D diblock copolymer on the phase separation kinetics of an A/B polymer blend are investigated by computer simulation. The rate of the phase separation process is significantly suppressed by the addition of block copolymers due to the reduction of the interfacial tension. This retardation effect is considerably dependent on the interaction energies (epsilon(AC) and epsilon(BD)) between the copolymer blocks and their respective homopolymers and the chain length of block copolymers. In the late stage of phase separation, the growth of the phase-separated domains obeys a dynamical scaling law in the cases of polymer blends both with and without a block copolymer. By a scaling analysis, we obtain universal scaling functions in the late stage which are nearly independent of the interaction energies and chain lengths of the block copolymer. This behavior indicates that the domain grows with dynamical self-similarity irrespective of the types of block copolymer used, although the block copolymer added Co the immiscible polymer blend reduces the growth rate of the phase-separated domains.