When expressing a thought, can とは思う and と思う be used interchangeably, or is there a more specific reason to choose one over the other?
If you heard about the difference between が and は, the existence of は there plays basically a similar role. (Maybe this question helps).
One of the functions/meaning of は is 取り立て, which means it indicates the speaker 'picks up' from multiple things.
と思う is the neutral to think that..... Adding は indicates that there are other things the speaker is thinking and from which the speaker 'picks up' what they say.
For example, 彼が悪いと思う means just I think he is to blame. 彼が悪いとは思う basically means the same, but implies the speaker is thinking something else too, and sounds like more nuanced (e.g., if the issue is between him and some other guy, he is to blame, but there are things the other shouldn't have done and the other is partially to blame too)