We present a theory of microphase separation of a diblock copolymer A(fN)B((1-f)N) and homopolymer hB blend in the strong segregation limit (chi N much greater than 10), where II is the Flory-Huggins interaction parameter. Homopolymer chains are assumed to be much longer than block copolymers : N-h much greater than N. In this case, homopolymers and diblock copolymers are separated in each elementary cell. We derive a general equation for the free energy and calculate it numerically for different microdomain superstructures : classical lamellar, hexagonal, and body-centered cubic (BCC) structures and also bicontinuous OBDD and gyroid structures. As a result : a phase diagram is constructed in coordinates (f, phi), where f is the diblock copolymer composition and rp is the volume fraction of the homopolymer in the blend. In particular, we predict a region of stability of one of the bicontinuous phases in the composition window, f = 0.62-0.66.