We study the metric backreaction of mass and angular momentum accretion on black holes. We first develop the formalism of monopole and dipole linear gravitational perturbations around Schwarzschild black holes in Eddington-Finkelstein coordinates against generic time-dependent matter. We derive the relation between the time dependence of the mass and angular momentum of the black hole and the energy-momentum tensors of accreting matter. As a concrete example, we apply our formalism to the Blandford-Znajek process around slowly rotating black holes. We find that the time dependence of the monopole and dipole perturbations can be interpreted as a slowly rotating Kerr metric with time-dependent mass and spin parameters, which are determined from the energy and angular momentum extraction rates of the Blandford-Znajek process. We also show that the Komar angular momentum and the area of the apparent horizon are decreasing and increasing in time, respectively, while they are consistent with the Blandford-Znajek argument of energy extraction in terms of black hole mechanics if we regard the time-dependent mass parameter as the energy of the black hole.