Line a large baking sheet with foil, then lightly oil foil.
Cook sugar with salt in a dry 10-inch heavy skillet over moderate heat, undisturbed, until it begins to melt. Continue to cook, stirring occasionally with a fork, until sugar is melted into a deep golden caramel. Immediately remove from heat and stir in almonds with a wooden spoon, then quickly pour onto baking sheet, spreading out mixture with back of spoon before it hardens.
Cool praline on baking sheet 5 minutes, then break into large pieces. Put praline pieces in a heavy-duty sealable plastic bag and break into smaller pieces with a rolling pin.
Bring jam, water, and lemon juice to a boil, stirring, over moderate heat. Add 1 1/2 cups blueberries and simmer until berries burst, 8 to 10 minutes. Add remaining 1 1/2 cups blueberries and simmer, stirring gently, just until heated through, about 2 minutes. Remove from heat and stir in almond extract.
Top scoops of ice cream with warm blueberry sauce and praline.