Preheat the oven to 350°F. Butter the base and sides of a 9x13-inch pan.
Drain a 29-ounce (822g) can of sliced peaches, reserving the juice. Set aside half of the peaches, then chop the remaining peaches into bite-sized pieces.
Whisk together the flour, baking powder, and salt until well blended.
Cream the softened butter and brown sugar in the bowl of stand mixer fitted with the paddle attachment until pale and fluffy, about 2 minutes. Scrape down the sides of the bowl with a flexible silicone spatula.
Add the eggs one at a time, beating well after each one. Add a tablespoon of the flour mixture with each egg to prevent the batter from curdling.
Pour in ¼ cup (60 ml) of the reserved peach juice, then add half of the flour mixture. Mix on low speed until blended, then add the milk and remaining flour mixture. Mix until well combined, but be careful not to overbeat or it will be tough.
Spread the batter evenly in the buttered pan and sprinkle the blueberries and chopped peaches on top.
Bake at 350°F for about 30 minutes, until it's a pale golden on top and springs back when gently poked with a fingertip. Let the cake cool a little on a wire rack while you prepare the jam and crumble.