Applying a Monte Carlo technique for the simulation of the dynamics of electrons in a semiconductor superlattice, we studied the action of a terahertz field on the motion of electrons by performing Bloch oscillations subject to inelastic scattering. We found that, due to polar optic phonon emission, the Bloch oscillating electrons can form k-space bunches phase locked to the terahertz field. For frequencies smaller than the Bloch frequency, this gives rise to the amplification of the terahertz field. Our study may contribute to the development of a Bloch laser for terahertz radiation.