What is the primary flavor of mousse aux cassis?

Mousse aux cassis is a classic French dessert that is specifically made using black currants, which is what 'cassis' refers to in French. The primary flavor comes from these dark berries, known for their rich, tangy taste that is both sweet and slightly tart. When incorporated into a mousse, black currants provide a balanced flavor profile that is essential for achieving the intended taste of the dessert. The light and airy texture of the mousse also complements the bold flavor of the black currant, creating a delightful experience on the palate.

Other berry options like raspberry, strawberry, and blueberry, while popular in various desserts, do not capture the unique taste of black currants that defines mousse aux cassis. Each of those berries provides distinct flavors that would change the essence of the mousse entirely, deviating from the traditional recipe. Thus, the correct answer is indeed black currant, as it is fundamental to the identity of the dish.