The original sentence is: The dark summer coat of an Arctic fox blends in with the snowless rocks.
This sentence is grammatically correct. It clearly explains that the Arctic fox's dark summer fur helps it blend in among the rocks that do not have snow. The structure, verb agreement, and word choice are all appropriate.
For example, the subject is the dark summer coat of an Arctic fox, which is singular, so the verb blends is correctly in the singular form. The phrase snowless rocks correctly describes rocks without snow.
So, you don't need to change anything! It's a well-written sentence.