To make the jam, mash the blackberries and sugar together in a 4-quart heavy-bottomed saucepan or Dutch oven over medium heat. Let the mixture come to a boil and cook for about 30 minutes total, stirring often (the timing will depend slightly on the shape of your pot).
When the jam is thickened and a thermometer registers 220°F, turn off the heat and ladle the jam into 2 pint jars or 4 half-pint (8-ounce) jars. It will keep in the refrigerator for up to 1 month. The recipe makes about 3½ cups, so you’ll have about a pint left over for later use.
To make the crust, combine the flour, almond flour and cinnamon in a food processor and pulse until evenly mixed. Sprinkle in the cubed butter, then pulse the mixture again until the butter is fully incorporated. Turn on the processor to run continuously, then pour the water in a thin stream and process for a few seconds, just until the dough begins to form a ball.
Turn out the dough onto a piece of parchment paper, and use a rolling pin to roll it into an 8-inch square. Transfer the dough to an 8-inch square baking dish, patting it in so that it completely covers the bottom of the dish in an even layer. Place the parchment on top of the dough, and pour on a set of ceramic pie weights or 1½ cups of dried beans.
Place the baking dish in the refrigerator for 20 minutes, and preheat the oven to 375°F.
While the oven is preheating, make the crumb topping. In the now-empty food processor bowl (there’s no need to wash it between uses), pulse together the flour, almond flour, sugar, cinnamon and salt until evenly mixed. Add in the cubed butter as you did when making the crust, pulsing until it is fully incorporated. Finally, pulse in the tablespoon of water, just until pebbly crumbs begin to form. Transfer the crumb topping to a bowl and place it in the refrigerator until you’re ready to use it.
Bake the weighted piecrust for 15 minutes, then remove it from the oven and carefully lift out the parchment with the pie weights on top.
Spread the jam onto the crust in an even layer, leaving a ¼-inch border of crust along the edges. Sprinkle the crumb topping over the top of the jam.
Reduce the oven heat to 350°F. Bake the jam bars for 35 minutes, until the crumb topping turns a light golden brown.
Let the bars cool to room temperature in the baking dish before slicing. They’ll keep on the counter, covered, for 3 days, or in the refrigerator for up to 1 week. You may also freeze them for up to 2 months.
*Cup4Cup flour is a gluten-free blend developed by Thomas Keller. It is sold at Williams Sonoma and other local retailers. The ingredients are cornstarch, rice flour, milk powder, tapioca flour, potato starch and xanthan gum.