Equivalent black carbon (eBC), elemental carbon (EC), and refractory black carbon (rBC) are widely recognized as distinct operationally defined metrics related to black carbon (BC). Building on the terminology framework of Petzold et al. (2013), this Opinion focuses on a practical question: how should empirical relationships among these metrics be represented and used? Conversion factors are often used for cross-metric harmonization, but their variability may also contain information on aerosol state and method response. We propose treating ratios such as eBC/rBC, eBC/EC, and EC/rBC as response ratios, namely derived diagnostic observables that can serve as empirical conversion factors when averaged for harmonization, but as indicators of changes in aerosol state and method response when retained as time-varying quantities. This dual role implies a practical equivalence trilemma: method specificity, state-independent numerical equivalence, and aerosol-state sensitivity cannot all be fully retained when cross-metric relationships are compressed into fixed conversion factors. The trilemma makes explicit a trade-off that the BC measurement community already navigates. It concerns cross-metric harmonization rather than within-method standardization or metrological traceability. We recommend transparent reporting of primary observables, conversion assumptions, and response-ratio time series in multi-method datasets. Retaining response-ratio variability alongside harmonized products would allow multi-method BC datasets to support both comparability and aerosol-state interpretation.