Ordered diblock copolymer blends have recently become the subject of tremendous research interest since they can be used to elucidate the intramicrodomain segregation of blocks differing in length, as well as to identify the molecular and blend parameters yielding phase immiscibility. In this work, we explore the influence of molecular architecture on block copolymer blend miscibility by examining an equimolar mixture of two symmetric styrene (S)/isoprene (I) block copolymers, one an SI diblock and the other an (SI)(4) octablock. Their molecular weights are identical, so that the ratio of block lengths is 4:1 SI:(SI)(4). While this ratio is expected to yield a single phase in diblock copolymer blends, transmission electron microscopy reveals here that the diblock/multiblock blend is macrophase-separated due to the Linear multiblock architecture and midblock conformations of the (SI)(4) copolymer. Electron tomography (3D imaging) permits direct visualization of connected SI and (SI)(4) microdomains at the SI/(SI)(4) interface at relatively high spatial resolution (ca. 3 nm). In addition, the presence of SI molecules in the (SI)(4) phase or (SI)(4) molecules in the SI phase frustrates SI lamellae, resulting in curved microphase boundaries.