Are you building a deck?
Do you just want to use Dek Blocks to place your hot tub on? If so, I'd be careful. It would really depend on the tub and the frame structure.
If you're building a deck, then I believe they work fine as long as the ground you are sitting on isn't too soft. But that would be the case regardless of whether you were putting a hot tub on the deck or not.
The weight of the hot tub will be distributed through your deck structure. You'll need to make sure you have an appropriate structure for the deck and you should be find.