Place the 2 tablespoons of water in a microwave-safe bowl and evenly sprinkle over the gelatin powder. Set aside.
In a large bowl, beat the cream cheese until smooth. Add in the sugar and beat until light and fluffy. Whisk in the whipped cream. Microwave the gelatin mixture until it has been completely liquified and quickly whisk into the cream cheese mixture until evenly incorporated.
Divide the mixture equally amongst three bowls. Leave one bowl untouched. Between the other two bowls, divide the cassis puree in a 50:100 gram ratio (or a 40:80 gram ratio, for a less tart flavour).
Scrape the pure cream cheese mixture into the bottom of the tart crust. Next, take a few tablespoons of the lighter colored cassis mixture and plop them over the bottom layer. Using the back of a knife or a chopstick, swirl the cassis mixture into the bottom layer. This will help to create a more seamless ombré effect. Pour over the remaining cassis mixture from the second bowl. Take a few tablespoons of the deeper purple cassis mixture and swirl them into the second layer. Pour over the remainder. Smooth the top of the mousse and refrigerate overnight.