Blueberry is a delightful berry to make liqueurs from. It is not something you commonly find in liquor stores. While it's easy enough to find blueberry vodka the color of crystal clear spring water, those products have more in common with a chemical factory than they have with a blueberry. As with most fruit I recommend using frozen organic blueberries. It is less expensive, and is generally a more consistent product year round. That said if you find a ridiculous farmer's market sale at the height of the season and you buy so many blueberries you couldn't possibly find fresh applications for all of them (yeah I can't really imagine that scenario either) then fresh work quite delightfully. Like most berries the infusion happens quickly, but time isn't really the enemy. There is no bitter or tannin quality that will take over long term and the alcohol prevents any decomposition in the berries.
I recommend a clear liquor because the berries have a delicate flavor that is served very well by a neutral spirit. The berry provides a deep rich flavor that takes very well to clear simple syrup and is very robust. The one thing I will say about blueberry liqueur is that it tends towards the syrupy very easily. Thankfully unlike most infusions there is very little in the berry that adds to the harsh alcohol bite of your final infusion so you can get away with less syrup when making your final liqueur. I recommend that you use 1 third sugar syrup and 2 thirds blueberry infusion. Like other berries I recommend just barely covering the berries with alcohol. Though the flavor is stronger than raspberry or strawberry, so if you want a large batch you can stretch it a bit. The blueberry is ever so slightly tart, and has a wonderful balance between delicate and robust. It mixes well with other berry flavors and wonderfully with citrus. I haven't tried it yet, but I'm guessing blueberries and cream would be a home run beyond compare. Happy infusing everyone.