  Kerr black holes coupled to quantized bosonic fields display a special
version of the Hawking effect, governed by the superradiance condition. This
leads to rapid growth of boson cloud through spontaneous creation, leading to
slowing down of the black hole, and detectable as growth of the black hole
shadow. This can be developed into a technique for searching or constraining
the existence of ultralight bosons. We study this phenomenon for spin-0 bosons
in the shadow of a black hole, with a detailed analysis of Sgr$A^*$, and put
estimates on the evolution time scales and subsequent change in the black hole
shadow features. Our study shows that there is a small window of parameters
where the increase of shadow of a supermassive black hole may be visible, but
only if the sensitivity of measurements increases from current 25 $\mu$as to
about 0.1 $\mu$as.
