We propose a unified phenomenological model based on relativistic gravitational impedance, Zeff = eta * G / c. The central observation is that the longitudinal mass flux Ig = d(gamma m0)/dt, proportional to the scalar product v dot a, defines a specific potential Phi_g = Zeff * Ig whose sign is set by v dot a. Longitudinal acceleration (v dot a > 0) corresponds to energy release with equation of state w -> -1 (dark energy phase), while deceleration (v dot a < 0) corresponds to absorption with w -> 0 (dark matter phase). The bare impedance Zg = G/c gives rho_g ~ 4e-36 J/m3 ~ 1e-27 rho_Lambda for the Milky Way, requiring amplification eta ~ 1e27 for cosmic acceleration and eta ~ 1e6 for the MOND acceleration a0 ~ 1.2e-10 m/s2, parametrized as a0 = eta * (Delta V)^2 / R_H. In the low-acceleration limit we derive v_c^4 = G M a0 from equilibrium v_c^2 = Phi_g. The model provides a single language for both dark components with falsifiable predictions for lensing near jets and Gaia streaming.