The phase behavior of binary blends composed of BABCB-type pentablock terpolymers and ABC-type triblock copolymers is investigated using the self-consistent field theory in the grand canonical ensemble. Specifically, the study is focused on how the simpler copolymers regulate the phase behavior of two sphere-forming multiblock copolymers of the type B(1)AB(2)CB(1) and AB(2)CB(3). For certain compositions, these two multiblock copolymers self-assemble to form mesocrystalline phases composed of binary A and C spheres with the ZnSC and ReO3 structures, respectively. It is discovered that the addition of symmetric ABC triblocks or AB diblocks to the two multiblock copolymers leads to the formation of different binary crystalline phases including symmetric binary crystalline phase of NaCl type for the A/C-component symmetric blends and asymmetric Cu2O, SnI4, TiO2, and CaF2 phases for the A/C-component asymmetric blends. In particular, the binary mesocrystals of the Cu2O and SnI4 structures observed in the blends of the tail-symmetric B(1)AB(2)CB(1) terpolymers and the AB diblock copolymers are new stable phases which have not been observed in the B(1)AB(2)CB(3) terpolymers melts. The theoretical results demonstrate that blending of block copolymers with specially designed multiblock terpolymers could provide an efficient route to fabricate binary mesocrystals.