In absence of visible light when we see any object or space to be dark (black), What is the wavelength we encounter so that the object or space looks dark to us?
Since there is no light, there is no wavelength to be measured. 'Black' is just your brain's interpretation to the absence of signals from the receptors in your eyes.
John Rennie put it better in this question.
IR rays (the body is black for us):
In all graphs there is a spectrum, that is black for us.