I read that when people say "Bless your heart", they actually think "you are an idiot and only Jesus can save you from your own stupidity". Is it true? No, that is not true. The phrase "Bless your heart" has nothing specific to do with Jesus. You are dealing with many subtle cultural nuances in consideration of the phrase "Bless Your Heart" and they are all so complex that it would take thousands of words to explain it to you. It can, in a narrow context of a certain type of a religious person, have some association with Jesus and the proposition that one should be viewed with contempt. However, it would be an overwhelming error in reasoning to suppose that such a restricted consideration of the phrase comprised a thorough answer to your question. Narrow parsing of words is seldom representative of any kind of a truth, and in the same regard, the employment of Sweeping Generalizations is not representative of any kind of a truth. The difficulty is that you are looking for a Black---And--White definition for the phrase, where Black--And--White definitions are not to be had. There are cultural contexts in which the phrase "Bless Your Heart" does convey that the target is a kind of unthinking idiot. That however, has nothing to do with either Jesus or Religion, as the phrase has secular as well as Religious usage. One can read all kinds of things. That is not necessarily an indication for a truth. Misunderstanding and errors in thought can be printed just as readily as a careful analysis of a subject. The English Language lends itself to broad (extremely broad usages). One of the difficulties which occurs with foreign students when they acquire some phrases, is that they errantly suppose that there are literal meanings which precise definitions for every phrase they hear. This is why students of English are well advised to learn literalisms, rather than subtle nuances of language. Nuances of language are the last thing that one learns. .

No I find that usually when people say it, they're trying to be nice and sweet - or they're trying to say that someone or something is cute e.g. "Oh you brought me flowers? Bless your heart" Usually old religious people do say it, but a young person might say it to be funny. Its not really 'preachy' usually