We have systematically mapped the phase behavior of a series of symmetric CE/C/E ternary copolymer/homopolymer mixtures, where C is poly(cyclohexylethylene) and E is poly(ethylene), identifying the location in composition of the technologically important bicontinuous microemulsion (BμE) channel as a function of diblock molecular weight. The lamellar-to-disorder transition, characterized by dynamic mechanical spectroscopy, small-angle X-ray scattering, and optical transmission measurements, exhibits increasingly second-order behavior as the BμE state is approached with increasing homopolymer content. Real-space transmission electron microscopy images obtained from rapidly frozen specimens evidence the development of large-scale fluctuating smectic correlations in the disordered state as the order-disorder transition is approached. This discovery provides fresh insights into the unexplained role of fluctuations in the formation of the BμE in ternary mixtures formed from binary blends of homopolymers that display an Ising-like critical point and a symmetric diblock copolymer governed by a weak, fluctuation-induced, first-order phase transition.