…muddling breaks up herbs, activating polyphenol oxidases (PPOs), vicious enzymes that cause fruits and herbs to turn brown and taste oxidized. That’s why muddled herbs never taste as fresh as you want them to. When you crush a piece of mint and it goes black and starts to taste like a swamp, PPOs are the main culprits. […] PPOs have two main enemies. The first enemy is booze. Forty percent alcohol by volume liquor will totally and permanently disable PPOs. The other enemy is the ascorbic acid (vitamin C) in lemon and lime juice—an antioxidant that retards their action.
— Dave Arnold, Liquid Intelligence: The Art and Science of the Perfect Cocktail
If you very lightly blend mint in booze, the mint stays green and fresh and the flavor gets really integrated.