A simplified version of the random copolymer lattice cluster theory has been applied to explain the variation in the nature of the phase transition (upper vs lower critical solution temperature phase diagram) with the microstructure of isotopic polybutadiene (PB) blends that has been observed by Hashimoto and co-workers (Macromolecules 1992, 25, 6078). This variation occurs within our theory when the temperature-dependent portion chi (h) of the effective interaction parameter chi (SANS) becomes negative (due to an increasing difference \y-x\ in the compositions x and y of the two PB random copolymers) and competes with a positive temperature-independent, "entropic" portion chi (s) of chi (SANS). The analysis is based on two important features. First is the calculation of chi (s) solely from the monomer united atom molecular structures, with no adjustable parameters. Second is the treatment of the van der Waals interaction energies in terms of united atom groups.