Is there a black hole in the sky?

The consequences of the hypothesis that a supermassive black hole can serve as a gravitational lens are analytically studied. It is shown that the presence of a black hole could be established by the unique property that it would appear against the microwave background as a black spot with a diameter of 0.1 arcsec or greater. The only instrument capable of either resolving the black spot or at least noticing it as a negative luminosity source is the Very Large Array.