In a medium bowl, add 1 cup sweetened condensed milk, ¼ cup key lime juice, and 2 egg yolks and mix to combine.
Remove the graham cracker crusts from their packaging. Layer in ~1 tsp of blackberry compote in the bottom of each tart.
Add in key lime mixture to the top of the shells. I find I use more than ⅛ cup filling, but less than ¼ cup filling. I also try to use a spoon or spatula to add the mixture from the outside in, otherwise the blackberry filling can creep up the sides. (Still delicious, but may not look quite as pretty! If it does, you can also run a toothpick through the mixture and make a swirled pattern as if it was intentional.)
Gently shake the shells to make the filling smooth and place the 6 tarts on a baking sheet. Bake for 10-15 minutes at 350° (mine take 13) until set. You can check by removing them from the oven and shaking them. You should see very minimal jiggling.
Wait for the tarts to cool, then garnish as desired! I used wild blackberries and basil leaves to garnish, but you can also use lime zest or lime wedges if you have those on hand.