Black holes are the most dense objects in the universe. Because of their density, they form extremely powerful gravitational fields. Black holes absorb all surrounding matter and energy within a certain proximity. For this reason, these celestial objects emit no light and therefore do not have a color. Astronomers can detect them, however, by monitoring the properties of the materials and energy surrounding them.

The electromagnetic spectrum describes the range of wavelengths and frequencies of different types of radiation. X-rays, radio waves and visible light are among the many types of radiation found on this spectrum. You experience the phenomenon of color when electromagnetic radiation of certain wavelengths reaches your eyes. Electromagnetic radiation travels faster than anything in the universe. It travels at almost 300 million meters per second (over 186,000 miles per second). Nevertheless, gravity affects electromagnetic radiation. Not even electromagnetic radiation can escape the gravitational force of a black hole. Therefore, you cannot actually see anything when you look at a black hole. No light, visible or otherwise, is emitted from the black hole itself.

More Info: sciencing.com