Ternary polymer blends, comprising two homopolymers and the corresponding diblock copolymer, have been examined by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). Two chemical systems have been employed: one consisting of polyethylethylene, polydimethylsiloxane, and poly(ethylethylene-b-dimethylsiloxane), and another containing polyethylenepropylene, polyethyleneoxide, and poly(ethylenepropylene-b-ethylene oxide). The molecular weights and compositions were chosen to emphasize the region of the phase prism dominated by the bicontinuous microemulsion (B muE) phase; the homopolymer molecular weights and volume fractions were approximately equal. The SANS intensity was compared quantitatively with the Teubner-Strey structure factor, and interpreted via the amphiphilicity factor f(a). The transition from a fully disordered mixture at higher temperatures to a well-developed B muE upon cooling did not correlate well with either the disorder line (f(a)=+1) or the total monomer Lifshitz line (f(a)=0). However, DLS provided a clear signature of this transition, via a distinct maximum in the temperature dependence of the dynamic correlation length. We hypothesize that this maximum is closely correlated to the homopolymer/homopolymer Lifshitz line. The structure of the interfaces in the B muE was further examined in terms of the curvature and the copolymer coverage, as functions of copolymer concentration and temperature.