Place fresh cherries in the work bowl of a food processor, and blend until a purée forms.
In a medium saucepan, combine cherry purée, frozen cherries, sugar, and corn syrup. Cook over medium heat, stirring frequently, until a candy thermometer registers 220°. Remove from heat. Add lemon juice, stirring well. Transfer marmalade to a heatproof bowl, and let cool completely. Cover and refrigerate for at least 1 hour or overnight.
Recipe by TeaTime Magazine at https://teatimemagazine.com/black-cherry-mousse/