Single-walled carbon nanotube (SWCNT)-based flexible thermoelectric films exhibit excellent electrical conductivity (σ) and high flexibility, making them promising candidates for wearable electronics. Black phosphorus (BP), a kind of emerging two-dimensional material, possesses high carrier mobility and exceptional thermoelectric conversion capabilities. In this study, BP nanosheets have been synthesized via liquid-phase ultrasonic exfoliation using N-methyl-2-pyrrolidone (NMP) and then composited with SWCNTs to fabricate freestanding BP/SWCNT composite films with enhanced thermoelectric performance. The optimized BP/SWCNT (4 wt% BP) achieves a maximum room-temperature Seebeck coefficient of -64.9 μV K-1 and a power factor of 345.7 μW m-1 K-2. Furthermore, a self-powered thermoelectric device is constructed by serially connecting three pairs of p-type SWCNT films and n-type BP/SWCNT films, delivering an open-circuit voltage of 4.4 mV and an output power of 303.4 nW under a temperature gradient of 60 K. Therefore, this work presents a facile and effective strategy for preparing n-type carbon nanotube-based materials with enhanced thermoelectric properties.