I am not a lawyer, but I can read what people write. See the following article from WaPo.
https://www.washingtonpost.com/news/volokh-conspiracy/wp/2017/07/06/it-is-actually-difficult-to-define-blackmail/?utm_term=.3c07ab692063
TL;DR: In order to be blackmail, there has to be a "lack of nexus." If I am in a dispute with you and I threaten to disclose the details of the dispute, there is "nexus" between my threat and what I may be asking for.