The Blue Moon is a lovely midcentury standard that fell by the wayside as crème de violette became difficult (or impossible) to get ahold of in the US. These days, three-quarters of a century later, there’s no shortage of fine brands producing excellent violet liqueurs.
Traditionally, the Blue Moon calls for Crème Yvette, a more complex liqueur flavored with berries, orange peel, honey and vanilla in addition to violets. It’s a lovely flavor, but it has the downside of making the drink more purple-red than blue.
You can use any violet liqueur you have handy – but if you’re a fan of floral cocktails, you owe it to yourself to find a bottle of Creme Yvette if you can.