Cutting into the block with a side grinder is the easiest way. You can also fish that wire through the block into a deep box and avoid that "physical damage" problem. Water shouldn't be an issue since block is not waterproof but thermal insulation/draft stopping may be an issue. The mounting pad usually has a 1/2" offset but if you shim it to the block with 3/8 ply instead of nailing to the furring it works out right for 1/8 panelling.
