The self-assembled structures formed in binary blends of AB/CD diblock copolymers are studied using self-consistent field theory focusing on cases with attractive A/C and repulsive B/D interactions. The attractive A/C interaction prevents macroscopic phase separation whereas the repulsive B/D interaction promotes B/D separation leading to the formation of complex hierarchical structures. The combination of these features makes the AB/CD blend an ideal model system for the study of hierarchical self-assembly. Our results demonstrate that the B/D separation leads to the emergence of modulated and alternate morphologies from the classical lamellar hexagonal spherical and gyroid structures. A common phase transition sequence from the mixed to modulated to alternate B/D structures is observed as a function of B/D interaction strength. The theoretical predictions of our model are consistent with available experiments and more importantly provide an interesting route for the engineering of hierarchically ordered structures using block copolymer blends.