The regions of the 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 BH population can have much larger influence. I discuss the effects of such emission, looking at a wide range of BH model populations. In all of them the BH radiation easily heats the IGM to 10^3-10^4 K, and achieves partial ionization. The most interesting consequence of this heating is that BHs are expected to induce a 21-cm signal (delta T_b ~ 20-30 mK at z<~12) which should be observable with forthcoming experiments.
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