A theoretical black object would absorb all visible light falling on it, and would therefore look like a total void, like the inside of a completely sealed unlit cave. Of course so called black object do reflect a little light.
However black object, including theoretically perfect ones, re-rediate some of the light falling on them. At room temperature, this radiated light is in the infra red and thus invisible. As the temperature of the object rises, the peak of the range of frequencies of the light is shifted towards the blue end of the spectrum. So at about 600 Celcius an object will begin to glow a dull red. As it heats, the colour changes through orange, yellow, white, blue-white and into the ultra-violet.