1. Pinch grapes to separate pulp from skins. Place skins in a bowl; set aside. Place grape pulp and 1/4 cup sugar in a medium saucepan; bring to a boil. Reduce heat; simmer 10 minutes or until seeds begin to separate from pulp.
2. Press pulp mixture through a fine sieve into a bowl. Discard seeds. Place skins, remaining 1/2 cup sugar, rind, juice, and pulp in saucepan; bring to a boil. Reduce heat; simmer until reduced to 2 1/2 cups (about 2 hours and 45 minutes), stirring occasionally. Pour into a bowl; cool. Cover and store in refrigerator up to 2 weeks.
Usually found only at farmers' markets, Concord grapes have a full, intense flavor. Concords are a slip-skin variety, which means the skins can be pinched off easily.