Just use plain Minute Tapioca, in any grocery store. Follow the directions on the box for thickening fruit pies.

Oh, do not stress. I use regular tapioca all the time for pies. Not even ground. I just soak it, maybe 3-4 tbsp for a regular pie, in warm/hot water, even better, the watery fruit juice and let it rest for a few hours. It will thicken, check it and add liquid as it does go thick quick.
Let it rest, even over night. It is my favorite thickener for any fruit pie or tart as it is neutral in flavor un-like flours. It does need a bit of heat but not hot like some thickeners.
It just needs a bit of 'time' and it will be super and soak up all your frozen fruit juice flavors.

I use minute tapioca in the little red box for blackberry pie. And not the homegrown ones, the little wild native berries that are worth their weight in gold here ;)

Made a deep dish blackberry this morning, I use approx. generous 1/8 C for a quart berries, closer to a 1/4 C for the 6 cups I used today. Of course, the juice in the berries will vary depending on the type, the little wild natives are not overly 'saucey' like a marion or tay berry may be.

I've never used tapioca flour. Stir the minute tapioca with your sugar, toss with the berries, and let the bowl of berries sit a few minutes while you make your pastry crust.

My neighbor (86 now, wonderful cook of many decades) always added a beaten egg to her blackberry pie!

For future reference, you should be able to find tapioca flour in Asian food stores. I bought some just a couple of weeks ago--in central Wisconsin.

never even thought to try there. Thanks, funny thing is I walked by one on the way from my car to the mexican place we had lunch at.

I guess time will tell. I made the pie last night, in a freshly cleaned oven, put it on an old cookie sheet for drip insurance and missed by a quarter inch to one side. I need a bigger insurance pan. The minute I smelled it I put down foil, but still ended up with the small ugly spots in my clean oven. Oh well I guess you have to break eggs .......... I still can't believe how juicy these berries are. tried to make a crisp out some of them last summer there was so much liquid it ended up being a mush not a crisp. I added a second batch of topping and rebaked it still didn't work out.

Hey, Blackberry is my absolute favorite. I'd eat it no matter...even if I had to sip it thru a straw!

I grind minute tapioca in the food processor but those little beads are tough, I still end up with a few little frog eggs in my pies, but I also like tapioca best for berry pies. I gave up on blackberry pie though, now I make blackberry cobbler, where the cakey/crusty part bakes on top of the juice! Blueberry pie for me is a real challenge too! Aesthetics only though, because once it's in a bowl and I start eating it, I don't care how runny it is!

If you frequently make pies, it may be worth investing in a pie drip guard. They have a center hole to let the heat get through to the pie. Mine is about 40 years old and like the one in the link below but without the non-stick coating.

Here is a link that might be useful: Pie drip oven guard

Only if I can get one bigger than 12 inches. I use stoneware pie plate and they have a lip on them just barely over 12 inches. I'll look around and see if they make one though. It's a great idea.

Macerate your fruit and cook down the juices to a thick syrup BEFORE BAKING. Just mix all the sugar and anything else together with the fruit and let it sit (in the fridge) for a couple of hours (I do it up to 24 hours with a firm fruit such as apple) and then drain off the juice, set the fruit aside, and reduce the juice until it is as thick as honey. Put the fruit in the pie as usual and add the reduced-to-syrup juice back in and bake as usual.

When making an apple pie, I also par-cook the apples. I've done apple pie that way for almost 50 years - it was taught me by an aunt after repeated goopy apple pie failures. Recently I discovered there is science behind this method (parcooking the apples) - it "sets" the apple so it won't get much softer in the pie. This is why I have for decades been able to make great apple pie using MacIntosh apples, which according to common knowledge are supposed to reduce to applesauce in a pie. They don't if you parcook them. They do get a little softer than most other varieties when cooked in a pie, but not unacceptably or even noticeably so unless you are doing a side-by-side comparison.

Anyway. I wouldn't parcook the berries as I think they may be too soft to be treated that way, but do drain off and reserve the juices and reduce it to a syrup. You should need little or no thickener.

As for grinding up tapioca pearls, you can do that - but I would use a spice grinder or coffee grinder (which of course has never seen a bean) - making sure the grinder is thoroughly cleaned out (run some rice through it several times and wipe out well to clean out any spice residue). A food processor just won't do a good job.

As someone else mentioned, you can find tapioca flour in an Asian grocery, but it could have any of several names:

Tapioca flour
Tapioca starch
Cassava flour
Cassava starch
Yucca flour
Yucca starch

Seems to me there is another additional possible name which I am forgetting, but hopefully that will cover your bases. They are all exactly the same thing.

I bet you could have found arrowroot powder somewhere also, that is often used as a thickener.

Oh, and I just looked it up - Tapioca/yucca/cassava starch/flour is sometimes labeled as "arrowroot" even though it isn't from the arrowroot plant. SO that's probably the 4th possible name I was trying to remember.