Catholicism never claimed to have direct divine revelation that blacks would never be admitted to the priesthood. Yes Mormonism later had a later revelation to reverse that policy (tho it took until 1978!) but like Romney himself, the Mormon church's changes in policy over the years have appeared to be based more upon expediency than any other factor. Unlike Romney, the Mormon church has claimed its policies are divinely inspired.
No, they just authorized slavery for for a few centuries (thank you Alexander VI). It was revoked as well, in 1993.