I don't know. For one thing, in my presence ripe blackberries don't last.

The only thing that springs to (my) mind is that it sounds like the setting of pectins in the juice. Green blackberries contain the most pectin, but the molecules are too tightly bound to use without special preparation. Part of the ripening process however, is to separate many such molecules from the cell walls, partly under enzymatic control, so the fragments of cell wall  that escaped with the juice separate out nicely and settle into a weak gel.

Which is all you need for a junket.

So that is my bet.

I don't know about blackberries but currants contain enough pectin to become stiff without adding anything and without cooking - with one problem: the pectin is concentrated in their skins which are quite difficult to really disperse amongst the rest of the berry stuff. But some blenders do it.

Yup, it is the pectin that does it. You can do exactly the same with rasberries.

As a matter of interest, pectin is recovered on a commercial scale from what is left over after citrus fruit and apples have had the juice extracted. The pectin is disolved out using a week acid and then precipitated out of the solution. Pectin is a soluble fiber and is used to produce  fiber drinks among other things.

It probably has an app for it.

If not, the iPhone probably does...

(sorry)