Two polymer blends of polybutadiene and polystyrene are investigated by small angle neutron scattering as a function of pressure and solvent content. We measured the thermal composition fluctuations, which we described with crossover functions. Whereas the crossover behavior of the susceptibility and the correlation length was the subject of Paper I [J. Chem. Phys. 116, 2229 (2002)], here we discuss the phase boundaries and the extracted mean-field effective Flory-Huggins parameter and radius of gyration for the asymptotic behavior at high temperatures. Using a lattice model and the lattice cluster theory of Bawendi and Freed, we developed some general expressions for the dependence of the Flory-Huggins parameter on the excess free volume and on the solvent content. A compensation of the free volume by the solvent molecules was observed. Finally, a discussion of the Clausius-Clapeyron equation allows for a check of the crossover function used to extract mean-field quantities for comparison with theory.