Self-consistent field theory (SCFT) is used to evaluate the excess free energy per unit area (i.e., tension) of different boundaries (or interfaces) between the coexisting phases of a block copolymer blend. In this first study of its kind, we focus on the boundaries separating the short- and long-period lamellar phases that form in mixtures of small and large symmetric diblock copolymers of polymerizations N-s and N-l, respectively, when Ns << Nl. According to strong-segregation theory (SST), the tension is minimized when both sets of lamellae orient parallel to the boundary, but experiments [Hashimoto, , Macromolecules 1994, 27, 1562] tend to observe kink boundaries instead, where the domains of one lamellar phase evolve continuously into those of the other lamellar phase. Our more refined SCFT calculations, on the other hand, do predict a lower tension for the kink boundary consistent with the experimental observations. For completeness, we also examine the boundaries that form when the short-period lamellar phase disorders, and again the SCFT results are in agreement with experiment.