Try Bitter Truth Violet Liqueur. I find it results in a pale blue color rather than the grayish purple that many of the violet liqueurs create.

The violet liqueur matters. Where I live there are basically 2 choices for the violet liqueur - Bitter Truth Violet Liqueur and Rothman & Winters Creme de Violette. Bitter Truth makes a beautiful, clear, pale blue dink while R&W makes a muddy purplish gray cocktail. They taste different too - I prefer the Bitter Truth for taste as well as looks in this drink. Also, fine straining the lemon makes the drink more clear.