A recipe for blueberry jam I have stars to run the berries through a food mill. Problem: I don't have a food mill. Can I just whiz them in my food processor instead?
You can but you will end up with a more textured jam than if you used a food mill.
If you do want the same silky smooth (and seedless!) texture you would get from a food mill, you can pass the blueberries through a fine mesh sieve after having puréed them in the food processor.
A food mill will remove the seeds and skin for you, basically straining the jam as you "process" the berries. If you have a China cap or mesh strainer, I would push the berries through it using a laddle after the food processor to remove the skin and seeds.
Food mills are inexpensive and an amazing addition to a kitchen. The food processor will not take the 'skins' away.