Small-angle neutron scattering experiments were conducted on a series of off-critical binary polymethylbutylene/polyethylbutylene (PMB/PEB) blends over a wide range of blend compositions, component molecular masses, temperatures, and pressures. The blends become more immiscible with either decreasing temperature or increasing pressure. A simple extension of the Flory-Huggins theory that accounts for finite volume changes of mixing (DeltaV) is presented. Our extension demonstrates the validity of the usual mean-field theory of scattering from polymer mixtures based on the random phase approximation (RPA) at elevated pressures. We use this framework to analyze the temperature and pressure dependence of the small-angle neutron scattering profiles obtained from binary PMB/PEB blends. We propose that the volume change of mixing is a linear response to the repulsive interactions between monomers. We demonstrate that off-critical PMB/PEB blends can be undercooled or superpressurized deep into the metastable two-phase region (e.g., up to 50 degreesC undercooling) without detectable signs of phase separation. The chi parameters and the statistical segment lengths obtained by fitting the data obtained in the metastable region are within experimental error of those determined from stable, single-phase PMB/PEB blends well-removed from a phase boundary. This indicates that the concentration fluctuations in the metastable region of the phase diagram have a mean-field character similar to those in stable, single-phase blends that have been extensively characterized by the RPA-based theory.