O.M.G.
Washed and dried fresh-picked blueberries, on a sheet pan w/ non-stick foil on the bottom, sprinkle with just a wee bit of sugar (not too much, you'll sweeten later), roast for 15-20 minutes at 425F until bursting and a bit juicy (it's easy to over-roast). If you over-roast, then add a handful of fresh berries to your dish while letting this cool. The residual heat will soften those, but still keep texture.
It'll thicken on it's own
Taste...and sweeten as needed. You can't really tell how sweet the berries are until they roast. Sweeten more if needed, a splash or two of lemon juice, and just a tiny tiny dusting of cinnamon. You don't want to TASTE the cinnamon, but when you taste the berries, you want them to have that warm taste. The umami of the blueberry sauce
Put on oatmeal, ice cream, mascarpone mousse, or you know, eat straight from the pan.
