Put the blackberries in a bowl, sprinkle with 1 tablespoon of the sugar, and twist the rosemary with your hands to release the essential oils and throw it on top. Gently toss the berries to combine and let them sit absorbing the flavor while you make the Muscat jelly.
Combine the Muscat, lemon juice, orange peel, and remaining sugar and in a heavy pot and place over medium-low heat. Simmer and stir, until little bubbles appear around the pot and the alcohol starts to vaporize; about 15 minutes.
Soak the gelatin sheets in cool water for 2 minutes to soften*, then remove them and squeeze out the excess water. Remove from the heat and fish out the strips of orange. Add the gelatin sheets into the hot wine and continue to simmer for 1 more minute, stirring to dissolve completely. Let the jelly cool slightly for 5 minutes.
* if using powdered gelatin, soak in 2 tablespoons cool water for 3 minutes, then add to hot wine mixture.
Fill 4 martini or wine glasses a quarter of the way with blackberries; carefully pour the Muscat jelly over the berries to fill the glasses to the top. Chill until thoroughly gelled and set, at least 4 hours or up to overnight. Garnish the Muscat jelly with whipped cream and mint; serve it with the remaining Muscat.