We would need a radio telescope, or one sensitive to even longer wavelengths.
Work Step by Step
The energy of a photon is given by E = hf, where h is Planck's constant. The light from the astronauts climbs the gravitational well of the black hole, losing energy. If the energy of light is lowered, as in traveling upward in a gravitational field, its frequency is lowered (the light is said to be red-shifted). Light signals from the astronauts would be red-shifted to the low-frequency, long-wavelength domain of radio waves, and eventually to even longer wavelengths.