This may be as simple as a synonym for hallmark, but with a negative connotation.
An example: A security consultant publishes an article on security practices that is so egregiously erroneous that it reflects poorly on 3rd parties who cite the article or author. The poor reflection is what I'm trying to describe.
Black mark is close, but I'd like to find a word taking a stronger stance - one could have multiple black marks that are overcome by strong qualities, while this word/phrase is effectively the end of the discussion.
Maybe deal breaker, but less vernacular.