I send out a blissey and use wish and then switch into a darmanitan that uses belly drum so after halving its HP will wish restore its HP so that I have max attack and full HP.
No it would not. Wish would heals the turn after. So Darmanitan would be healed as it switches in (before you use belly Drum).
Darmanitan does not need Belly Drum anyway. It has sky high attack and s=using would only mean death, even at full health because its so frail.