Looks like a ...
...crater filled with the "Dark Dune Material."
Same color as the dune field that was seen during the descent video and surrounding rock layers look like a crater. Other, smaller examples are nearby, as well as dark patches more irregular in shape, probably relative topographic lows acting as sand traps.
Thanks for prompting me to Google Mars, I'been avoiding it, lest I get sucked in. That's some fantastic topography ahead. I can only imagine the discussions going on with the planning team.