In the pre-reionization Universe, the regions of the intergalactic medium (IGM) which are far from luminous sources are the last to undergo reionization. Until then, they should be scarcely affected by stellar radiation; instead, the X-ray emission from an early black hole (BH) population can have much larger influence. We investigate the effects of such emission, looking at a number of BH model populations (differing for the cosmological density evolution of BHs, the BH properties, and the spectral energy distribution of the BH emission). We find that BH radiation can easily heat the IGM to 103–104 K , while achieving partial ionization. The most interesting consequence of this heating is that BHs are expected to induce a 21-cm signal ( δTb∼ 20–30 mK at z≲ 12 ) which should be observable with forthcoming experiments (e.g. LOFAR). We also find that at z≲ 10 BH emission strongly increases the critical mass separating star-forming and non-star-forming haloes.