Extension of the sensitivity of single-photon detectors beyond the near-infrared is key to enabling advances in astronomy, free space communications, remote sensing applications, and security. We have developed and validated an optical fiber-coupled superconducting nanostrip single-photon detector based on tungsten silicide with blackbody-limited performance and low timing jitter in the mid-wavelength infrared region. The detector performance is enabled by a differential impedance-matched architecture, compact cryogenic self-aligned fiber packaging compatible with fluorozirconate fiber, and an optical stack optimized for maximum absorption at 2.9 µm wavelength. The device system detection efficiency and specific detectivity from 1.06 µm to 3.5 µm wavelengths are presented. Characterization of device timing jitter, recovery time, and polarization extinction ratio in the aforementioned wavelength range is measured. The packaged device demonstrates performance exceeding commercially available mid-wavelength infrared detectors, and a specific detectivity at 2.9 µm greater than 100 % of the blackbody-limited performance of an ideal photovoltaic detector at this wavelength.