An Achilles heel is a weakness, something that makes you vulnerable, either physically or emotionally.
"Blondes were his Achilles heel" = "He had a weakness for blondes"
The term comes from the character Achilles in Greek mythology. According to legend, his mother dipped him as a baby in the river Styx, a magical river, which made Achilles invulnerable to arrows, swords, spears, etc. His one weakness was his heel, because his mother held him by the heel when she lowered him into the river. This meant that the magical river never touched his heel.
This proved to be his doom, for he was killed by a poison arrow through his heel during the battle of Troy.