1 - "I am very blessed": When you say someone is 'blessed', means that some divine power gives you blessings and because of that you're fortunate because of that extra power. 2 - "I am very luck": When you say someone is 'lucky', means that things happen in favor that person without any reason. In my point of view the first sentence 'I am very blessed' gives a more religious idea, therefore, I think this to be more formal than the second one.