To recognize or sense a weakness or vulnerability (especially a new one) in an opponent, adversary, or subject over which one is trying to gain an advantage. The phrase implies that such a weakness will then be acted upon to gain victory or advantage. The visiting squad is starting to look tired, and the home team scents blood. Look for them to try to close out the game. Do not give any indication that we are willing to settle. If we do, the attorneys will scent blood and we won't get what we want.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
In a competitive situation, if you scent blood, you sense a weakness in your opponent and take advantage of it. Right-wing parties, scenting blood, have been holding talks aimed at building an alternative coalition. Note: You can also say that someone gets a scent of blood. The market has got the scent of blood and, having sniffed it, they are going for it.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012