Blackbeard's name is Marshall D. Teach, and it's very unlikely he's a yonkou. For one thing, he only recently left Whitebeard's crew. And he's trying to build up his reputation as a strong pirate so that he can join the Shichibukai. Indeed, the marines at Mariejoe hadn't heard of Teach. A Yonkou would be known by the marines.

I don't know if Dragon is actually a villain. They haven't said much about what he's done. But being a revolutionary alone doesn't make him a villain. The Strawhats are responsible for four revolutions that I can think of. (Axe-Hand Morgan, Arlong, Wapol, Enel) And they declared war on the World Government on Enies Lobby. But they definitely aren't villains.

Considering what the government did to Robin, I'm inclined to think that Dragon was somehow involved with Gol D. Roger, that he knows how to read the Poneglyphs and read the Rio Poneglyph, and that he's spreading the word.