Ezra of Ithaca (NY, USA) asks :-
What is the colour black?
John Campbell, a physicist at the University of Canterbury, responded.
Blackness is merely the lack of reflection of the visible light to which our eyes respond.
We usually regard matt black paint (blackboard paint) as very black but it isn't a perfect absorber of light. Make the following object. Take a shoebox. Paint the inside white or dayglow orange. At one end cut a hole out about 5cm in diameter and paint the whole end matt black. Stick a piece of black velvet to cover half of the end, triming round the hole so it is clear but one half is adjacent to velvet and the other half to black matt paint. Which appears blacker? The hole by far. The paint looks almost shiny.
The black matt paint reflects more light than velvet which reflects more light than the hole. Light entering the hole undergoes many reflections before it gets out again so although the inside surface of the box is quite reflecting, after many reflections almost all of the light has been absorbed.
The light is absorbed in several ways, the main one being in exciting the electrons in the atoms of the surface. These dont reradiate but their energy is dispersed as heat, the surface heats up. That is why solar panels are painted black, or with crystalline needles sticking up like velvet fibres so any reflected radiation has many reflections before it can escape back to where it came from hence almost all is absorbed. The thermal energy of the surface diffuses into the surface or radiates away as infrared radiation and the whole thing comes to thermal equilibrium. Pour more light intensity on to it and the system heats up more. Eventually it can be hot enough to vapourise the paint.