Polymeric bicontinuous microemulsions (BμE), found in well-designed ternary blends of two homopolymers and a diblock copolymer, have been extensively studied in the bulk, for example, as versatile templates for the synthesis of nanoporous materials. However, there have been few reports regarding BμE-forming blends as films and the potential impact of confinement on the morphology of such blends. We have investigated the morphology of ternary blends of polyethylene (PE), poly(ethylene-alt-propylene) (PEP), and poly(ethylene-b-ethylene-alt-propylene) (PE-PEP) on a variety of substrates. The films were rendered nanoporous by selective extraction of the PEP component, which also created contrast for scanning electron microscopy (SEM). Blends that form BμEs in the bulk were found to undergo an evolution of morphology from a BμE to a macro-phase separated state, induced by the segregation of blend components to the film interfaces. The dynamics of the transformation are accelerated by decreasing film thickness. The results presented indicate that BμEs can be kinetically trapped on arbitrary substrates, which has important implications for the production of bicontinuous, nanoporous films.