The viscoelastic properties of near critical entangled polybutadiene (PB)/polyisoprene (PI) blends were investigated in oscillatory shear above and below the lower critical solution temperature (LCST). The terminal loss modulus of a near critical PB/PI blend above and below the LCST is well described by means of a log additive mixing rule. A single master curve in the loss modulus of the critical blend exhibiting WLF behavior was obtained above and below the LCST by using the empirical time-temperature superposition (tTS) principle. However, the storage modulus above the LCST deviates from both the tTS principle and the log additive mixing rule. The phase-separated PB/PI blends within the linear viscoelastic regime display higher than expected values of storage modulus at low frequencies, due to the interfacial tension between the two phases of the blend. This increase causes a discontinuity in the temperature dependence of the storage modulus at low frequencies. The discontinuity occurs at a frequency-dependent temperature that extrapolates in the limit of zero frequency to the cloud point measured under quiescent conditions by optical microscopy. The evaluations of interfacial tension from low-frequency linear viscoelasticity are compared with the expectations of Helfand-Tagami theory. The rheological determinations of interfacial tension are within a factor of 2 of the expected values for all compositions except those near the phase-inversion point.