Thanks all for the comments. Here is the recipe I used, adapted from the one in Bon Appetit. Although the recipe does not mention it (my bad), we did include an egg white in the processing as we do with nearly all our sorbets. For those who wince at the idea of a chocolate-orange sorbet, Baskin-Robbins used to have a delicious chocolate-orange sherbet, and I've always loved chocolate coated candied orange peels.
I might add that the mixture was smooth (by taste) before chilling. I failed to check it after chilling on its way into the ice cream maker. I believe we first encountered the use of egg white in sorbet from Pierre Franey, but I would have to look that up to be sure.
2/3 cup sugar
Finely grated zest of two oranges
4 cups water
1 tablespoon instant coffee crystals
1/2 cup frozen orange juice concentrate, thawed
1 pound bittersweet (not unsweetened) or semisweet chocolate, finely chopped
Process sugar with orange zest until fully incorporated.
Bring orange-infused sugar, water, and coffee crystals to boil in large saucepan over medium-high heat, stirring until sugar dissolves. Mix in orange juice concentrate. Reduce heat to low. Gradually add chocolate, whisking until smooth. Chill uncovered 4 hours, stirring occasionally.
Process sorbet mixture in ice cream maker according to manufacturer's instructions. Transfer to container. Cover; freeze at least 6 hours and up to 3 days.
Adapted from recipes in Bon Appetit and Sunset Magazine