Put the lavender into a tea-ball straining spoon and use that to stir the blueberry mixture, leaving it in the pan to steep the whole time. Put all other ingredients, except butter, in a saucepan and simmer until reduced by half.

After the blueberry mixture cools, blend it up with the stick of butter.

Put it into a zip-lock bag pushing it down in the bottom to form a log, and put in the refrigerator until firm.