Well, I think the difference is in the intent. Being blunt just means you are being direct and getting straight to the point. I appreciate that because it's likely that I won't understand properly if people aren't direct enough. Being 'snarky', however, means you are deliberately trying to be unkind or disrespectful. I don't often realise when people are doing this unless it's really obvious. I'm more likely to be offended by people not being direct enough, particularly if they know I won't understand what they mean but say something in a vague way anyway.