"Dark spots on the wall" and "black pictures"

0Kenneth Clark wrote:02br
02br
00"Impressionism is a short and limited episode in the history of art... We may therefore survey, with some detachment, the gains and losses of the movement.02br
02br
00There was the gain of the light key. Ever since Leonardo attempted scientifically to achieve relief by dark shadows, pictures have tended to be 01b01u00dark spots on the wall02u02b00. Those who have visited the Naples Gallery with remember with what relief, after wading through three centuries and about thirty rooms of 01b01u00black pictures02u02b00, one comes on the rooms of antique painting, still fresh and gay in tone. They are only journeyman's work, far poorer in execution than the great canvases, but they are a delight to the eye."02br
02br
00Please help me to understand what the author mean by "dark spots on the wall" and "black pictures."02br
02br
00Cadzao0-