Cook Mixture / Infuse
In a saucepan warm the milk, sugar, one cup heavy cream, salt, sage, blackberries, and tea bag. As the mixture heats, start squishing down the blackberries, but be careful not to pop the tea bag.
Once the mixture is hot and steaming, and the blackberries have mostly turned to a fruit mush remove from heat, cover, and let stand for an hour to infuse the tea flavor.
Use a mesh strainer to strain the mixture into one of the bowls. Be sure to strain out the tea bag, any seeds from the blackberries, and any chunks of fruit that haven’t broken down. Discard what has been strained out. Rinse out the strainer and saucepan, you will need both again.
Prepare Ice Bath
Fill the largest of the bowls with ice. Place a slightly smaller bowl on top of the ice and pour the remaining heavy cream into it. Set the strainer over the top.
Finish Cooking The Mixture
Pour the strained blackberry mixture black into the saucepan and rewarm it over low heat.
In the now empty bowl, whisk together the egg yolks, then slowly pour a small amount of the warm blackberry mixture into the yolks, whisking constantly. Do this a second time, then scrape the warmed yolks back into the saucepan. (If the eggs get hot too fast they will cook so be sure to add the gradual amounts and mix well.)
Cook the mixture, on low or just above (but not too hot!), stirring frequently with a heatproof spatula, until the mixture thickens and coats the spatula. Note: Don’t heat the mixture too quickly or the eggs will scramble in the ice cream.
Immediately strain the mixture into the cream that is waiting over the ice bath, then stir the mixture until cool and well mixed.
Churn / Freeze
Refrigerate the mixture overnight. The next day churn it in your ice cream maker following the manufacturer’s instructions. When finished churning, transfer to a storage container, cover and freeze until firm.