Bridge columns, as core load-bearing and force-transferring components in bridge structures, are highly susceptible to partial damage and even global failure when subjected to close-in explosions. Therefore, analyzing the damage characteristics of reinforced concrete (RC) bridge columns under such blast scenarios and developing corresponding damage assessment methods are significant. In this study, a high-fidelity three-dimensional numerical model of an RC bridge column was developed in ANSYS/LS-DYNA and validated against field blast experimental measurements. Using the verified model, the typical failure processes and damage mechanisms of the column were systematically investigated. According to the extent of cross-sectional damage, the failure modes of the column were classified into three types: non-spalling, spalling, and breaching. Additionally, the influence of initial axial load was considered, and the regions for different failure modes were analyzed. Finally, on the basis of the analysis of failure modes and residual capacity, a material loss-based damage index P was proposed, and the relationship curves between residual capacity-based damage indices D and P under different damage modes were established. Using the control variable method, the relationship between these two indices under the influence of a single parameter was further explored, and empirical formulas were derived to express the correlations among longitudinal reinforcement ratio, damage index D, and damage index P under both non-spalling and spalling damage modes.