Crush the Oreo cookies into fine crumbs using a food processor for a smooth base. Melt the vegan butter and combine it with the crumbs in a bowl until evenly mixed. Press this mixture firmly into the bottom and up the sides of a tart pan, ensuring a compact layer. Place the crust in the freezer for 15 minutes to set while you prepare the mousse.

In a blender, puree the blackberries and blueberries until smooth, adding a touch of sweetness if desired. For a seedless mousse, strain this mixture through a fine mesh sieve. In a mixing bowl, combine the berry puree with vegan cream cheese, sugar, and vanilla extract, blending until creamy and uniform—this delightful mixture forms the base of your No-Bake Blackberry Mousse Tart.

In a separate bowl, use an electric mixer to whip the chilled non-dairy cream until stiff peaks form, taking about 3-5 minutes. Once whipped, gently fold the cream into the berry-cream cheese mixture until well combined, being careful not to deflate the whipped cream, which adds a light and airy texture to your mousse filling.

In a small saucepan, combine the agar-agar powder with the non-dairy milk and bring to a gentle boil over medium heat, stirring continuously for about 2-3 minutes until the mixture thickens slightly and is dissolved. Allow it to cool for a few minutes before incorporating into the mousse mixture to ensure a smooth blend.

Fold the cooled agar-agar mixture into the berry mousse, mixing gently until fully combined. Be cautious not to overmix, as you want to keep the airy texture of the whipped cream intact. This step is crucial for ensuring the mousse sets beautifully in the No-Bake Blackberry Mousse Tart.

Remove the crust from the freezer and pour the mousse filling into it, smoothing the top with a spatula for an even finish. Cover the tart with plastic wrap and place it in the refrigerator to chill for at least 4 hours, or overnight is even better to allow it to firm up perfectly.

Once set, carefully unmold the tart from the pan. To enhance its presentation, top it with fresh berries, a sprinkle of chocolate shavings, or whipped coconut cream. Slice and serve your stunning No-Bake Blackberry Mousse Tart chilled, delighting your family and friends with this effortless and vibrant dessert.