Preheat oven to 350F. Set springform pan aside.
In a pan over medium heat, add butter and allow to melt completely. Once melted, add brown sugar and stir. Allow mixture to bubble for 1-2 minutes, stirring occasionally.
Pour mixture into bottom of a tightly fitted springform pan. Quickly arrange pineapple slices over the mixture. Add blueberries in the center of pineapple rings and around the gaps as desired. Set aside.
In a mixing bowl, cream butter and sugar together until blended together.
Add eggs and vanilla extract. Continue to beat mixture together.
Gradually add the flour, baking powder, baking soda, and salt to mixture. Beat on medium speed to combine.
Add in milk and pineapple juice to mixture. Beat together for a minute on high speed. Make sure to stop and use a spatula to wipe down the bowl, then beat again for another minute.
Pour batter evenly onto the pineapple/blueberry layer in the springform pan.
Bake at 350F for 30-45 minutes, or until a knife/toothpick inserted in the center comes out clean with few crumbs.
Allow to cool for 15 minutes minimum. Then release the springform pan from the cake (use a knife to loosen the edges if necessary). With the bottom pan still on the cake, gently and carefully invert the cake onto a platter (it will still be hot, so be careful). Wait 5 minutes, then gently lift the bottom pan from the top of the cake.
Serve this cake fresh as it's best when hot out the oven!