Add all except a small handful of blackberries to a small saucepan on medium heat. Cook for about 10 minutes, stirring occasionally and breaking apart the blackberries with your spatula, until mixture resembles a chunky jam.
Meanwhile, mix together the rest of the ingredients (oats, chia seeds, milk, protein powder) in a bowl.
Spoon a layer of the bowl mixture into the bottom of each mason jar. Follow by spooning in a layer of the blackberry mixture. Repeat this layering until all the ingredients have been used up.
Top with reserved whole blackberries. Chill for at least 2 hours before eating.
Notes
The sky is the limit on your fruit choices. You can just as easily sub blueberries, raspberries, strawberries, or any other soft juicy fruit into these protein overnight oats. That goes for frozen fruit, too! For larger fruits like strawberries, just make sure to slice them first before tossing them in the saucepan, to help them break down faster.
Although I've provided volume approximations for the ingredients, I always weigh my items on a food scale for my recipes. So, following the weight measurements will yield the best results. Plus you will have less dishes to clean if you pour ingredients directly instead of measuring them out with a spoon or cup. When I was done, I only had to wash a saucepan, a bowl, and a spatula.