Freezing has always turned my wild blackberries red too. But usually the color returns upon cooking. So my jam looks like blackberry jam.

It varies by variety; my triple crowns stay darker when frozen than other varieties, but all have a noticeable purplish tinge with red highlights.

I saw a web page once that included a column on the post-freezing color per variety, but I can't come up with it now.

I have always slow-frozen berries (room temperature, then into the refrigerator for hours, then into the refrigerator freezer, then into the deep-freezer. That may help keep the berries dark, check out
Freezing rate and color change of blackberries

Here's a pic of my Triple Crown on their way to becoming jelly. They were never frozen, but had spent a few days in the fridge.

That's what my pie looks like! (color-wise, I mean....)

Well it looks like Triple Crown is not typical. I looked at my wild blackberry jam again, and it is also red, not as much as this though. But I have made jam from store bought blackberries and it was darker. So i think TC is not typical. I'll know more next year when i have a number of my own cultivars fruting.

I've never had anything nearly that red in any of my triple crown processing during freezing, juicing, pureeing, baking, etc. All my product is very dark.

I don't know how you can get that red of a blackberry, must be alchemy of some sort.

I just pulled some TC out of the freezer, the circular pattern is ice that had formed on the cover. That's as red as mine get.

{{gwi:125517}}

Is there any ascorbic acid or lemon juice being applied pre- or post-freezing to the berries? Anthocyanins, that contribute to the dark color of blackberries, can turn pink in an acid environment.

I used a small amount (1/2 tsp?) of grated lemon peel in my pie. But I have to say that the berries have a goodly amount of acid as it is. As a retired chemistry teacher I'm not discounting the potential effect, but it seems like a drop in the bucket compared to the existing acid. Of course, if the bucket is already full....

Which leads me to wonder if the acidity of the berries themselves is the cause. I live in a cool summer area and although I attempted to pick the berries at their best, I can't say that each was perfectly ripe or that the really ripe ones were as sweet as they might be somewhere warmer.

I didn't apply anything to mine. They were probably a bit under-ripe, as SWD started hitting them bad and I decided to use what I could. Hopefully it isn't the SWD worms which give it the color (uch...).

Here is a picture of the berries before being boiled.

It's the use of under ripe berries as to why they are so red. Good for cooking all the same. Anyway I think that may be why.