We investigated the phase behaviors of block copolymer (BCP) blends composed of the weakly interacting (with no specific interaction) polystyrene-b-poly(n-butyl methacrylate) (PS-b-PnBMA) and polystyrene-b-poly(n-hexyl methacrylate) (PS-b-PnHMA), using small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and depolarized light scattering (DPLS). A miscible phase between the PnBMA and PnHMA blocks in the BCP blends allows the various composition-dependent phase behaviors, which display a lower disorder-to-order transition (LDOT) to an order-to-disorder transition (ODT) on heating as the blend composition varies from PS-b-PnBMA to PS-b-PnHMA. In between these two extremes, intriguingly, the miscible PS-b-PnBMA/PS-b-PnHMA blends display a closed-loop phase transition consisting of a LDOT and an upper order-to-disorder transition (UODT).