So why don’t ground blind manufacturers add a rain fly, like on camping tents? The problem with a rain fly is that it will flap in the wind and spook game from coming anywhere near the blind. This defeats the purpose of the blind in the first place.
If you really want to make your blind watertight, you might want to try using a seam sealer or waterproofing spray. Keep in the mind that any sealant product you use will leave behind an odor. To get rid of this odor, let the blind sit outside for a few days until the smell is gone.
If you leave your ground blind out overnight, use tie-downs to keep the sides taut and prevent water from pooling in pockets on the roof or other places. If enough water builds up on your tent, it can cause damage to the tent material or poles.