B substituting O in antiferromagnetic Cr₂O₃ is known to increase the Néel temperature, whereas the actual B dopant site and the corresponding functionality remains unclear due to the complicated local structure. Herein, A combination of electron energy loss spectroscopy and first-principles calculations were used to unveil B local structures in B doped Cr₂O₃ thin films. B was found to form either magnetic active BCr₄ tetrahedra or various inactive BO₃ triangles in the Cr₂O₃ lattice, with ${\sigma }^{\ast }$ and ${\pi }^{\ast }$ bonds exhibiting unique spectral features. Identification of BO₃ triangles was achieved by changing the electron momentum transfer to manipulate the differential cross section for the 1s-${\pi }^{\text{*}}$ and 1s-${\sigma }^{\text{*}}$ transitions. Modeling the experimental spectra as a linear combination of simulated B K edges reproduces the experimental ${\pi }^{\text{*}}$ / ${\sigma }^{\text{*}}$ ratios for 15–42% of the B occupying the active BCr₄ structure. This result is further supported by first-principles based thermodynamic calculations.