This paper investigates the influence of concrete type, conditioning temperature, and conditioning duration on the durability of concrete-encased basalt fiber-reinforced polymer (BFRP) bars. Two types of concrete were used to cover the BFRP, having a nominal cylindrical strength of 40 MPa: ordinary Portland cement concrete (CC) and slag-fly ash (1:1) blended geopolymer concrete (GC). The concrete-encased BFRP bars were kept in saturated conditions for durations of up to 6 months at temperatures of 20 and 40 °C. The durability was evaluated by testing for tensile strength and moisture absorption. Experimental test results revealed that having the BFRP bars in CC led to up to 61% higher tensile strength retention than counterparts in GC. This was associated with 86% less moisture absorption in the former specimens. Subjecting the samples to higher conditioning temperature (40 °C) caused a significant tensile strength reduction, with superior losses of up to 43% exhibited in those encased in GC for 6 months. Conversely, BFRP bars in CC displayed minimal strength loss (less than 5%) under the same conditions. These findings suggest that BFRP bars encased in slag-fly ash blended geopolymer concrete are more susceptible to strength degradation at high temperatures and longer exposure durations compared to those placed in ordinary Portland cement concrete.