Conventional blankets consist of a layer of rubber and fabic and dont have a compressible layer in them as 'compressible' blankets do. Metal backed blankets I believe are mainly used with large very high speed Newspaper presses.
They are "Compressible Blankets" specific for High Speed Newspaper Offset Presses, nobody should be using "Conventional Blankets" these days, especially on Web Offset Presses.