Need both types of mixes.
I suggest using milk instead for both mixes.
Bake the brownie mix for 7 to 10 minutes at the recommended temperature for the mix you used.
You want the top of the brownies to be firm.
While brownies are backing, make the blueberry muffin mix. Again, I suggest using milk instead of water for the mix.
Take out the brownies after 10 minutes. Pour the blueberry muffin mix over the brownies. You will not be able to smooth out the mix, so spread the mix as you pour / spoon it out of the bowl.
Turn the oven to the recommended temperature for the muffins. Bake for about 20 minutes. Let cool for 10 minutes. Enjoy a unique dessert.