We consider here a polymer blend made of two crosslinked polymers A and B of different chemical nature, in the presence of a common solvent assumed to be good for both species. To study the solvent effects on the microphase separation of the mixture, we first apply the so-called blob model which is a direct consequence of the renormalization theory. In this model and at large scales compared with the screening length xi of the semidilute regime, polymer chains A and B are viewed as sequences having blobs of size xi as subunits. Within this model, we calculate the structure factor enabling us to derive all microphase critical properties. The main result is that, the swelling effects simply lead to a renormalization of these properties, and that the renormalization factors are powers of the monomer concentration. Quantitatively, such a renormalization is in agreement with the fact that, in presence of a good solvent, the mixture is more compatible. The blobs picture is a mean-field approximation, to go beyond this approach which neglects the strong fluctuations of the density near the critical point, we reconsider the problem using the renormalization-group techniques. We show that these strong fluctuations lead to a microphase critical behavior characterized by nonclassical exponents, which are found to be identical to those of the Ising-type system.