It's not like the block placing ability can't be balanced in the game. They can limit the number of blocks you put, make them easily destructible, or even make them disappear after a set time. Just because an idea hasn't been implemented in Smash yet doesn't mean it will be game breaking.
If they limit the block placing to 5 or so, make it so the blocks have to be placed next to ground or each other, and make it so you have to wait until the blocks disappear or you mine them to get them back, then it should be fine. Could still be useful, as you could make a ledge and lead opponents to it to try and get them closer to the blast zone, or make a wall and combo them against it. It wouldn't be campy at all, not anymore campy than what's already done anyway.
EDIT: By the way, Steve can abuse the blocks with a set time by placing the blocks close to the blast zones in midair and get rid of the rest except the ones close to the blast zones, and then Steve can replace one of the blocks that has disappeared already.