Arrange the chicken thighs in a single layer in the slow cooker.
In a medium bowl, combine the honey, tamari, blackberry jam, oyster sauce, olive oil, garlic, onion and crushed red pepper flakes. Pour this mixture over the chicken thighs. Cook on low for 4 to 5 hours until the chicken is fully cooked.
Remove the chicken from the slow cooker (reserving the liquids) and place it on a plate. Using two forks, shred the chicken.
In a small mason jar or tupperware, combine the cornstarch with 3 tablespoons of cold water. Shake well until fully combined.
Transfer the liquids from the slow cooker into a small saucepan set over medium-high heat and whisk in the cornstarch mixture. Bring the sauce to a boil and reduce until thickened enough to coat the back of a spoon, about 5 minutes. Pour the sauce over the chicken, tossing to combine. Serve topped with sliced scallions and sesame seeds.