Without having your setup locally, it isn’t easy to test, but you could try something like this:
${ $index + 1 }. {=title} ${ $settings.isBlockVisible == "0" ? "[HIDDEN]" : "" }
That’s a mix of UFM ({ expression }) and UFMJS (${ expression })
{= property }: UFM, shorthand for {umbValue}
${ property }: UFMJS, javascript-like expressions such as math and booleans