When someone you are competing against begins to exhibit a weakness, what would you do? You would make use of the weakness to your advantage. This is what the expression `to smell blood’ means — to make use of someone’s weakness or problems to your advantage. It is also possible to say `scent blood’.
*When our main bowler was injured, the opposition began to smell/scent blood.
When a shark smells blood, it goes into a frenzy. It attempts to find out where the smell is coming from. It tries to locate its prey making use of its amazing senses, and when it finds the wounded animal, it attacks. The wounded animal is at a disadvantage, and the shark makes use of this weakness to kill it.