Multilayer black phosphorus (BP) nanoplatelets of different thicknesses were prepared by the liquid phase exfoliation method and deposited onto yttrium aluminum garnet substrates to form saturable absorbers (SAs). These were characterized with respect to their thickness-dependent saturable absorption properties at 3 μm. The BP-SAs were employed in a passively Q-switched Er:Lu2O3 laser at 2.84 μm. By using BP exfoliated in different solvents, stable pulses as short as 359 ns were generated at an average output power of up to 755 mW. The repetition rate in the experiment was 107 kHz, corresponding to a pulse energy of 7.1 μJ. These results prove that BP-SAs have a great potential for optical modulation in the mid-infrared range.