Ghika (a mid-twentieth century Greek artist) painted a landscape with a black sun. Magritte talked about "the black sun of melancholy" but so far as I know never painted one. With apologies to both, I have tried to supply the deficit.
The message is of course the beginning of Donne "A Nocturnal upon S. Lucy's Day, being the shortest day". The theme of this poem is that his beloved has died, so for him the darkness will be permanent.