Blackberry rhubarb crisp is what we made (using up the last of last summer's blackberries from the yard that we froze):
THAT sounds good!
I like rhubarb pie not combined with anything else like strawberry (yuck), but if it has to be mixed with anything, then fresh blackberries are it.
I did make a rhubarb blackberry pie last Winter and it was incredible. I tried a second time and it wasn't as good. I don't use recipes so that's the reason.
When making a pie I buy a premade but fresh crust, and use lemon, butter, light brown sugar (no white) and a little flour in the fruit mix. When possible I use organic fresh ingredients, second choice is frozen ingreds. Never canned.