The caption to this THEMIS image of Hellas refers to indistinctness in the image and attributes it to the thicker atmosphere over Hellas:

PIA04055: Winter in Hellas Basin
http://photojournal.jpl.nasa.gov/catalog/PIA04055

I don't remember seeing this used before as a cause of reduced seeing over low elevation regions on Mars. Has this been found in prior Viking or MGS images?
Since clouds are a common feature over Hellas another possibility is that the bluriness is due to low lying fogs. In this winter image such fogs would probably consist of ice crystals.
However, Hellas was found to be one of the areas that could allow liquid water for up to 30% of the Martian year. Fogs might suggest water lying close to the surface in Hellas. Then during the spring/summer months this water might melt to form liquid water.
It would be interesting to find out it there is a seasonal cycle to the GRS hydrogen readings in Hellas. If there is, this might suggest a hydrological cycle of evaporation/sublimation and precipitation of water/snow over Hellas.

Bob Clark