> The sole purpose of blockchain is to prevent double spending without a trusted party

NIST's Blockchain Technology Overview is really good explainer:

* https://csrc.nist.gov/publications/detail/nistir/8202/final

See especially Figure 6 ("p. 42", 53 of the PDF), which is a flow chart to help you decide on whether blockchain matches one's use case. Extracted:

* https://imgur.com/a/RlUj9Ed