Prepare the blackberries: Combine the blackberries, sugar and cassis in a bowl. Let sit at room temperature for 30 minutes, then crush with a fork.
Meanwhile, make the caramel sauce: Combine the cream and vanilla pod and seeds in a small saucepan and bring to a simmer over medium heat. Remove from the heat.
Whisk the sugar and 1/2 cup water in a medium nonreactive saucepan over high heat. Cook until dark golden brown, 10 to 12 minutes. Remove from the heat and slowly add the vanilla-infused cream (it will splatter), stirring constantly with a wooden spoon. Return to the heat and cook until smooth. Remove from the heat, discard the vanilla pod and stir in the butter and salt.
For each sundae, place a scoop of ice cream in a parfait glass and top with caramel sauce, blackberries and pecans. Repeat 2 more times and top with whipped cream.