Why would the Blues do that?

No way you get a 1st for Perreault. Maybe a 3rd with some retained salary, lol.

Because the Blues want Perreault.

I would not stretch to a first. I would make it a conditional first.
1st conditional on Perreault hitting 50 pts. If he fails to hit 50, but manages 0.5 pts/game with at least 50 games played, it becomes a 2nd and 3rd. If he fails to hit 0.5 pts/game or play 50 games it becomes only a 2nd.
if you want to trade him straight up, its a 2nd and 3rd.
Right idea though
But trade him to someone outside the division (BUF/MTL)

If cap is so important to the Jets why even worry about the return? You could just give him way and be better off, if you really wanted Stasny.

You could get more for perreault. There would be some sort of bidding war. you could get a 2nd and 3rd easily