Here's my response to these verses:
I always assumed it was symbolic. Like, they became wicked so their countenances became dark. We all know people who have made terrible choices and their faces and countenances change
Let's assume it's literal. Just because that was the curse anciently, doesn't mean this applies now. I don't know any LDS people, including leadership, that teaches that this is black people, that black people have a curse from God, etc. In fact, leadership has spoken out about misinterpreting these verses to mean that and have said thats wrong. In the Bible, God cursed people with physical infirmities to symbolically show wickedness. This is similar and again doesn't apply outside of that time and those peoples.
There are black Mormons. People can be racist, so some mormons are racist just because Mormons are people. But I know plenty of black Mormons. I almost married a black man. My best friend married a black Mormon. My aunt married a black Mormon.