A Monte Carlo simulation has been performed on the dynamics of phase separation of A/B immiscible polymer blends to which various C-b-D diblock copolymers with repulsive interactions between the blocks and homopolymers were added. For comparison, the results for a blend containing a block copolymer with attractive interactions are also presented. It is found that even the block copolymer with repulsive interactions between the blocks and the homopolymers still has a retardation effect on the phase separation. This effect may arise from strong interfacial activity that is due to the competitive interactions of the blocks with the different homopolymers. The phase separation proceeds by following the usual spinodal decomposition process, and thus the scaling law between the first moment of the structure factor and the time, q(1)(t) proportional to t(-1/3) can be applied in the later stage of separation. By analyzing the changes in the linear dimensions and the contact numbers of the block copolymers with the phase separation time, we investigate the effects of the type of block copolymer interaction and its relative strength on the interfacial activity and the consequent retardation of separation.