No matter what the farm stand down the road thinks, blackberries are not black raspberries, no more than lemons are yellow limes. Black raspberries don't taste like blackberries. They don't look like blackberries. They're wild raspberries that are so dark purple they look black, and they're often (as in the case of these) less tart than red raspberries - but unlike golden raspberries, which are milder and more subtle than red raspberries, they have a deep winey-berry-y flavor. Black raspberry ice cream is a popular flavor in New England (I've found a lot of locals don't realize it isn't a standard flavor nationally).
For the pie, there's nothing between the crusts but black raspberries, sugar, and cranberries that I added both for tartness and to add some thickening.