A self-consistent field theory (SCFT) technique for calculating the compositions of coexisting phases in melt blends of reversibly bonding block polymers is presented. This new method is obtained by deriving a canonical model for reaction equilibrium and incorporating it into the previously reported Gibbs ensemble unit-cell SCFT for investigating macrophase separation. We use our new technique to attack the phase behavior of an incompressible melt blend of AB diblock copolymers that reversibly react at their B termini with monofunctional B homopolymers to produce longer ABB diblock copolymers. We find that reaction equilibrium favoring the product polymer stabilizes a rich variety of ordered phases with little macrophase separation. Reaction equilibrium favoring the reactants, however, leads to macrophase separation over a broad range of compositions.