A Rock Blanket is a geology tool used for shaping and polishing rocks.
The Shawshank Redemption
When Andy's cell is searched Hadley comes across a "rock blanket" which Andy explains with his hobby rock shaping and polishing, showing them the chess pieces. Thankfully, no one asks him how he does the shaping - after all he had to hide the rock hammer which Norton later finds in the Bible.
The "toss in the cells" was not a real inspection. It was an opportunity for Norton to size up Andy. Since it wasn't a serious inspection, Norton neither was interested in conducting a thorough review of Andy's cell nor was intent on discovering the root cause to Andy's rock carving hobby (knowing nothing about it, the rock blanket would seem good enough). It's reasonable to assume that rock hammers, tools, and cutting devices were prohibited in the prison after that ... to prevent prisoners from trying to escape using that method again.