Underscore in src/bin, i.e. 'src/bin/foo_blah.rs'

I am looking at Package Layout - The Cargo Book

IntelliJ recognizes sources of name src/bin/foo.rs but not src/bin/foo_blah.rs

Is there some convention against the usage of underscore as a top level name in src/bin ?

Nope, it's perfectly acceptable to have an underscore in your binary's name. This is probably something related to IntelliJ.