Perhaps it would be wise to consider that there is nothing inherently wrong with a raven, nor is there anything inherently wrong with the color black. Seeing symbolism in a color when a passage does not even mention a color leads only to nonsense... nowhere does Genesis 8:6-7 mention the color 'black'!
But what about the dead animals that the raven ate which symbolize sinful desires?
Even if there were dead animals floating on the water, the raven might not have eaten them. Remember that in the beginning, God gave to people and animals only plants for food (Genesis 1:29-30). It was not until after the flood that God allowed people to eat meat (Genesis 9:3). Thus, there is a possibility that God also did not allow animals (including ravens) to eat meet until after the flood.
But if the raven does not symbolize an unbelieving heart... what does it symbolize?
Perhaps the best conclusion is that the raven in Genesis 8:7 does not symbolize anything. Perhaps Noah just wanted to see if the earth was dry.
Is it possible that Scripture is simply stating what happened during the flood? Just think, the flood wiped out every living creature from the face of the earth. For almost a year, all the people and animals of the earth (not sea creatures) were contained in the ark. Perhaps Noah's decision to release the first living land animal into the world after all that destruction (in order to see if the earth was dry) was an event significant enough to mention!
Noah Sends Out An Unsymbolical Dove (Part 1) - Genesis 8:8-12
The Chronology of the Flood: The Raven and Dove - Genesis 8:6-12
Did Noah Gather "Vegetarian" Food? - Genesis 6:21
The First Sight Of The Post-Flood World - Genesis 8:5