If my team needs an instruction on Markdown editing on Hugo with Blackfriday (not MMark), which one would be appropriate as reference for a Markdown spec?
You are right: There is no spec for Blackfriday in special.
For special cases you just have to try or skim the docs, e.g. ^superscript^, ~~strikethrough~~, ~subscript~, definition lists …
For Hugo 0.58 the implementation of goldmark is planned. Goldmark is fully compliant with latest CommonMark spec and extensible! I am definitely planning to switch.