The Thing still wins
The Xenomorphs are chumps compared to the Blob and the Thing. The Blob still has no answer to the thing assimilating it.
Xenomorph blood would likely hurt the blob and the thing. But no enough to kill either. After the Thing assimilates an alien, it could become immune to it anyway.
So this fight takes place during AVP2?
The Thing would destroy the Alien. And the Predators.
Especially after it has access to the lush vibrant climate. Plants. Insects. Lizards. Mammals. Xenomorphs.
The Thing would learn to breed with itself within hours once it absorbs both sexes of a single species. And even Jurassic Park style frog monosex breeding theory could bring good results. Regardless, once the Thing breeds with itself you have an army of Things in Colorado.