Preheat your oven to 350°F. Spray a 9x13 baking pan with cooking spray.
Add the pineapple. Pour the entire can of crushed pineapple — juice and all — into the prepared pan. Don't drain it. That juice is going to soak up into the cake mix as it bakes, and that's what keeps everything moist and delicious. Spread it out evenly across the bottom of the pan.
Add the blueberries. Give your fresh blueberries a quick rinse and check for any stems. Scatter them right on top of the pineapple layer. You want enough blueberries so that you'll get a little bit in every bite — anywhere from 1 to 2 cups depending on your preference. If you're using frozen blueberries, you can add them straight from the freezer without thawing.
Sprinkle the cake mix. Open your box of yellow cake mix and sprinkle the dry mix evenly over the fruit layers. Use your hands to spread it out so it covers the fruit as evenly as possible. Don't stir anything — just let it sit right on top.
Add the pecans. Scatter your chopped pecans evenly over the cake mix. If you like bigger pieces, leave them in larger chunks. If you prefer a finer crunch, chop them up a bit more. This is totally optional — the cake is delicious without them too — but they add a really nice crunch and flavor.
Drizzle with melted butter. Pour your melted butter slowly over the entire surface, going back and forth to cover as much of the cake mix as possible. This is the step that gives you that beautiful golden, crispy topping. You want almost all of the cake mix to be covered — it's okay if a few little spots peek through, but do your best to get the butter everywhere.
Bake for 45–50 minutes until the top is golden brown and you can see it bubbling around the edges. Every oven is a little different, so start checking at 40 minutes. If you want yours a little darker on top, give it a few extra minutes.
Let it cool slightly before serving. It's best when it's still warm — scoop out a generous serving and top it with a big scoop of vanilla ice cream. That combination of warm, bubbly cake with cold ice cream is just perfect.