They probably do and should regret that trade, but as I stated above, this is a very different trade proposal. Mantha is every Habs fan's (and probably management's) wet dream. Romanov is a very good prospect, but he's far from a can't miss, top pair defensive prospect. If they want Mantha, it starts with Romanov, and they add from there.
If the Blues are looking to offload salary, I would think they would trade one of their bad contracts. To move multiple bad contracts in one trade, it would cost way too much. I'd be asking for Robert Thomas or multiple 1st's.
Bokk was traded to Carolina in the Justin Faulk trade.