Mean-field theories (Flory-Huggins, RPA) predict that the critical repulsive interaction parameter (chi(c)) for polymer-polymer phase separation is independent of the monomer sequence distribution. In this work, we generalize the RPA to account for sequence distribution effects on the spinodal by including fluctuations in both the composition and its conjugate field. We calculate chi(c)(p) for a blend of homopolymers and regular multiblock copolymers with 2p blocks and find an architecture-dependent shift of relative order N-1/2. This shift either increases or decreases as p is increased (corresponding to a more evenly distributed copolymer), depending roughly on whether the homopolymer is shorter or longer than the copolymer. Fluctuations also shift the critical composition away from the Flory-Huggins value, even in asymmetric homopolymer/homopolymer blends.