This black spot is one of seven possible entrances to subterranean caves identified on Mars by Glen Cushing, Tim Titus, J. Judson Wynne and Phil Christensen in a paper they presented at the Lunar and Planetary Science Conference in March

The hope for the HiRISE images was that we could see some details from inside the hole.

HiRISE is a very sensitive instrument, and Mars' dusty atmosphere scatters quite a bit of light around, so there is certainly light entering that cave hole and bouncing around the interior. But it seems that the cave is so big and so deep that almost none of the light that enters the cave comes out.