Coat the inside of a 1.5 quart casserole with non-stick spray. Place blackberries in the dish.
In a medium bowl, combine cake mix, butter, lemon zest and nutmeg. Use a pastry knife to cut the butter into the other ingredients until it resembles coarse crumbs.
Spread the crumb mixture over the blackberries in an even layer. Cover the dish loosely with foil - do not seal.
Pour ,uc>1 cup of water in the Instant Pot and insert the trivet. Carefully lower the baking dish on to the trivet.
Secure the lid, making sure the vent is closed.
Using the display panel select the MANUAL or PRESSURE COOK function*. Use the +/- keys and program the Instant Pot for 10 minutes.
When the time is up, let the pressure naturally release for 10 minutes, then quick-release the remaining pressure.
Carefully remove the dish and set it under the broiler, uncovered, until golden, 3-5 minutes.
Serve warm with whipped cream or vanilla ice cream.
Notes
*The MANUAL and PRESSURE COOK buttons are interchangeable.