I really need an item for my blastoise.
That's not really enough information to go on - it also depends on moves and stats. But Blastoise tends to be a defensive Pokemon, so Leftovers is generally a good option.
Depends what moves and nature it has.
Is it an offensive blastiose, is it a deffensive blastoise?
Since its a blastoise am guessing its deffensive(offensive blastoise have water spourt- like mine).
Id say LEFTOVERS.