In a medium saucepan, combine 1 1/2 cups of the orange juice and the sugar over medium heat. Cook, whisking often, just until the sugar is dissolved. Remove from heat, stir in the remaining orange juice, cover, and refrigerate until well chilled.
Transfer the mixture to an ice cream maker and churn until starting to set, about 20 minutes. Pour in the chopped dark chocolate and continue to churn for another 5 minutes or frozen through.
Transfer to a freezer safe container if not serving right away. Store for up to a week or two.