The stability of a binary polymer blend with respect to macro- and microphase separation is considered. It is shown that the loss of stability of the spatially homogeneous state of the blend via microphase separation resulting in formation of an equilibrium microdomain nanoheterogeneous structure is possible. The conditions for this effect are (i) a significant negative entropic contribution to the Flory-Huggins chi parameter, i.e., an entropic contribution to miscibility, and (ii) the nonlocal character of this contribution with a high value of the nonlocality radius. Both these conditions can be met for miscible polymer blends somewhat above their glass transition temperature. It is argued that the nanoheterogeneity recently observed in polystyrene-poly(vinyl methyl ether) blends can be explained by this effect.