Spread blueberries a baking sheet lined with parchment or silpat, turn oven to 350 and roast until berries just begin to exude their juices. Remove the blueberries and replace with raspberries. Turn off oven and let the raspberries “roast” for another 5-10 minutes, until soft. Mash berries in separate bowls until no big chunks remain. Strain raspberries through a sieve to remove the seeds, this is a bitch to do but adds oodles to the enjoyment later. This is the point where you might want to add a little lemon or sugar (or both) to the mashes depending on how sweet or tart the berries you have are. I added about a teaspoon of the rum to my raspberries as well.
In a saucepan over medium-low heat, bring half and half to a simmer with 1/3 cup sugar and the salt. Simmer, stirring occasionally, until sugar dissolves. Transfer to a bowl, stir in liquor of choice and vanilla and place in an ice bath in the refrigerator to chill. You don’t have to do both the ice bath and the fridge but the combo speeds the process up just that much more and you know you want this ice cream ready an hour ago.
When cold, pour mixture into ice cream machine. Add blueberries and churn according to manufacturer’s directions. Transfer to a container and swirl in the raspberry puree. Freeze until solid, at least 2 hours.
Let sit at room temperature for 5 to 10 minutes before serving, or in refrigerator for 15 to 30.