Screenshot from Mars Trek
This enigmatic crater lies in southern Utopia Planitia at 28.72⁰ N, 119.96⁰ E, is about 10 km wide and has a min. height difference between its floor and rim of ca. 1650 m.
The crater's floor has the lowest elevation of minus 6 km on Mars between its latitude and that of the northern rim of the Hellas basin.
Could it be that the structure and/or the composition of the surface material inside the crater cause it to be radiation-absorbent ?
Screenshot from Mars Trek
The image above shows the second "dark hole crater" in Utopia Planitia at 39.5⁰ N, 105.4⁰ E, about 900 km to the northwest of the previous one. This one is about 12 km wide and here you can clearly see its flat floor.(flat, according to Mars Trek)
As far as I know, these two craters are of the few ones on Mars that look like dark holes with bright rings.