Board Thread:Show-Related Questions and Answers/@comment-25536775-20170501202433/@comment-25926288-20170501203131

Don't ya know. It make sure you become a hero!

Seriously though, it's either because Blue enchanted it with light magic (The Jekyll Serum was propped up by Rumple's dark magic) or because the sword means automatic battle win when you strike something with a mortal blow.