  In a variety of mechanisms generating primordial black holes, each black hole
is expected to form along with a surrounding underdense region that roughly
compensates the black hole mass. This region will propagate outwards and expand
as a shell at the speed of sound in the homogeneous background. Dissipation of
the shell due to Silk damping could lead to detectable $\mu$-distortion in the
CMB spectrum. While the current bound on the average $\mu$-distortion is
$\left| \bar{\mu}\right|\lesssim10^{-4}$, the standard $\Lambda$CDM model
predicts $\left| \bar{\mu}\right|\sim10^{-8}$, which could possibly be detected
in future missions. It is shown in this work that the non-observation of
$\bar{\mu}$ beyond $\Lambda$CDM can place a new upper bound on the density of
supermassive primordial black holes within the mass range
$10^{6}M_{\odot}\lesssim M\lesssim10^{15}M_{\odot}$. Furthermore, black holes
with initial mass $M\gtrsim10^{12}M_{\odot}$ could leave a pointlike distortion
with $\mu\gtrsim10^{-8}$ at an angular scale $\sim 1^{\circ}$ in CMB, and its
non-observation would impose an even more stringent bound on the population of
these stupendously large primordial black holes.
