Is it ok to blow hot air under the floor?
While this should work, I'd be worried about losses, including 1/2 the heat going down to the slab instead of up to the floor.
A simpler solution would be to insulate. If that involves ripping up the floor then it would probably not make sense. But you may be able to blow in insulation through the same holes where you are thinking of blowing in hot air, or possibly cut a few holes in hidden places - e.g., inside closets - where the fixup afterwards doesn't have to look perfect.
If you do end up blowing air under the floor, make sure that you have an easy way to turn it off in the summer - you really don't want air conditioning under your feet!