An educated guess.

Dictionary of Electronics [Collins]:
1. black level = a voltage level in a TV video signal that results in the electron beam being cut off. Signals below this level cannot produce any effect on the screen of the cathode ray tube.
2. black level clamp = a circuit used to ensure that the black level in a TV pricture signal is maintained at a steady level that determines the overal brightness of the picture.

Dictionary of Multimedia [Peter Collins Publishing]:
clamper = circuit which limits the level of a signal from a scanning head or other input device to a maximum before this is converted to a digital value; used to cut out noise and spikes.

See also: http://www.freepatentsonline.com/6940548.html .