Stir the mixture every few minutes, allowing it to chill until it’s slightly wobbly but not yet firm, adding more ice to the ice bath if needed. You can use an instant-read thermometer if you like. When the Jell-O mixture is around 45 to 55ºF, it’s at the “lightly gelled” stage. This should take about 30 minutes.
Add the sour cream and blueberries:
Remove the bowl with the Jell-O mixture from the ice bath. Add the sour cream and blueberries to the lightly gelled Jell-O mixture, stirring until the sour cream is fully incorporated, with no big lumps. It’s fine if a few streaks of white remain.
Mold the Jell-O mixture:
Pour the mixture into a large (10 1/2-cup) Jell-O mold or bundt pan. Tap the pan gently on the countertop a few times to make sure that the mixture has settled evenly, then cover with plastic wrap and refrigerate to chill until fully solid, at least 8 hours or up to overnight.
