Not to be pedantic about it but there actually was slavery (to your point though, not nearly as widespread as it was in the East) Brigham Young declared it legal right after a black member from Boston made the trek into Utah. As soon as he heard Brigham Young say that he packed up and left. He only stayed for like a month.

Edit: I’m not certain off-hand if it was made legal-legal or if Brigham just said it was doctrinal, imagine the effect would be pretty much the same either way at the time.