Thermal composition fluctuations were studied in the blend of polybutadiene (PB) and polystyrene (PS) in temperature and pressure fields using the technique of small angle neutron scattering. The blends had a critical composition and were mixed with different amounts of the nonselective solvent ortho-dichloro-benzene. In this part we discuss the critical behavior as measured along the critical isochore and interpret the results in terms of a crossover function describing the transition between mean field and 3D-Ising behavior. In particular, we will discuss the Ginzburg number and the critical amplitudes of the susceptibility and correlation length. For the Ginzburg number Gi we found a much stronger increase with solvent content as predicted while the pressure fields always lead to a decrease of Gi as expected. In "pure" PB/PS blends we found a further crossover to a so-called renormalized Ising behavior; such a crossover proposes density fluctuations of the total polymer concentration near the critical point. This crossover was not observed in the blend-solvent mixtures.