We present the surface structures formed under the void microchannels of an elastomeric mold when thin films of polystyrene/polybutadiene (PS/PB) blends are annealed with the mold placed on the film. Exactly opposite types of structure result under identical annealing conditions, depending only on the height difference in the as-cast thin film between the PS and PB phases. Recessed patterns form under the microchannel by long-range interactions when this height is more than 60 nm; the usual protruding patterns result in the void microchannels due to the short-range interactions of capillarity when the height is less than 40 nm. The height of fully developed meniscus of the mobile PB in solid PS is believed to be the factor that determines the type of pattern formation.