Block copolymers are well-known for their large number of microphase morphologies on mesoscopic length scales. After a short review of the different morphologies observed in binary block copolymers and ternary triblock copolymers, the self-assembling in blends of different block copolymers into common superlattices is discussed in detail. Besides similar morphologies known for pure triblock and diblock copolymers. the blends can also show new morphologies. Examples of such new morphologies are periodic non-centrosymmetric lamellae and multiple gyroid interface structures. The discussion of the superlattices is primarily based on investigations by transmission electron microscopy (TEM), which are supplemented in a few cases by small angle X-ray scattering (SAXS) or results from computer simulations.