  Primordial bubbles that possibly nucleate through quantum tunneling during
inflation in a multidimensional potential might have left some relic detectable
at the present time. These bubbles turn into black holes during the radiation
era, which may account for the LIGO black holes, supermassive black holes, and
may play an important role in dark matter. Typically, these black holes are
surrounded by an energy deficit in the form of a spherical sound wave packet
propagating outwards. In this work we study how this perturbation of the cosmic
plasma dissipates before the time of recombination, leading to spectral
distortions in CMB. We find that there may exist some rare regions on the last
scattering surface containing huge black holes, which have produced potentially
detectable point-like signals of $\mu$-type distortions.
