Preheat oven to 350°. Bake pecans in a single layer in a shallow pan 8 to 10 minutes or until toasted and fragrant, stirring halfway through. Cool 15 minutes.
Stir together mayonnaise and next 2 ingredients. Stir in pecans and cheese.
Spoon mixture into a plastic wrap-lined 4-cup ring mold with a 2 1/2-inch center. Cover and chill 2 hours.
Unmold cheese ring onto a serving platter. Discard plastic wrap. Stir together blackberry preserves and basil. Fill center of ring with blackberry preserves mixture. Serve with crackers. Garnish, if desired.