I said I would check a dictionary. Frank: "Open and sincere in expression; straightforward: 'made several frank remarks about the quality of their work.'" Blunt: "Abrupt and often disconcertingly frank in speech: 'People [in the Western US] are blunt with one another, sometimes even cruel, believing honesty is stronger.'" So there you have it. Blunt means frank, but not just frank: "abrupt and often disconcertingly frank."

Being frank and being blunt are similar, but I would not say the same. To me, being frank is being honest, but in a considerate way. Being blunt is just saying the truth without much consideration of someone’s feelings, just saying things the way they are. I don’t know if I’m making sense, but here are a couple of examples I can think of: To be blunt, that color looks bad on you. To be frank, I think cooler tones compliment you better.

It's subtle. It's subtle enough that I'm going to answer first, but check a dictionary. And first I'm going to introduce an idiom: a "little white lie." A "little white lie" is a lie that is told to avoid hurting someone's feelings. An example would be telling someone "you look great!" when you think they only look OK. To be frank means to tell the truth, not telling little white lies, not softening the truth, but without meaning to hurt someone's feelings. To be blunt means telling the truth in an unpleasant way when you could be nice and still be frank. The school drama department is putting on a production of "Hamlet." You tell the teacher in charge that you are going to try out for the part of "Hamlet." The teacher knows that's a crazy idea, you aren't anywhere near good enough to play that part. The teacher answers: (Little white lie) "That's great! Tryouts are Thursday at 3 pm." (Frank answer) "I'm sorry, but aren't ready for that role. Why don't you consider a smaller part?" Blunt response: "That's crazy. You aren't good enough. Don't waste your time or ours."