Black Mark Meaning

(idiomatic) something that negatively affects someone's reputation

Example: 2011 April 6, quoting Paul Simpson, “Management after the sack”[Aimee Lewis], BBC Sport:
  "It taints your CV as people look at it and see what's gone on and it does put a bit of a black mark against you," says the man who, in 2006, was head hunted by Preston only for his tenure to be brought to a halt 17 months later.