If you are blameless, you did nothing wrong, so people cannot fairly say the trouble is your fault. It is a rather formal word: you meet it in news, in official reports, or when someone is defending a person's character. Friends chatting would more often just say it wasn't their fault. Though actually, blameless does not mean nothing bad happened. An accident can still hurt people, and you can still be blameless if you were careful and honest. People also talk about a blameless life, meaning someone who has stayed out of trouble for a long time, not only in one moment. It can sound a little old-fashioned, and sometimes a little too tidy — as if nobody is ever quite that clean — but the core idea is simple: no blame sticks to you.