a question about Ringwraiths

this may seem silly, but i'm only on chapter 7!!

ok how come Frodo didn't know that the "black riders" weren't the ringwraiths? Gandalf told him about them. How that there are 9 of them and that they were the mortal men that got the rings. How could frodo not relize who they were? Also this question has nothing to do with this thread: How did Smeagol turn into a grey slimy creature? I mean i look up at that icon and that doesn't look like a hobbit creature, like gollum once was, does that mean that if frodo wears it long enough that he would look like him?