Heat oil in pan over medium heat til it shimmers. Add onion and sweat til softened, 3 minutes. Stir in 1 cup blackberries, broth, brown sugar and chipotle. Bring sauce to simmer and cook til blackberries are softened and mixture slightly reduces. This should take about 2 minutes.
Whisk together lime juice and cornstarch; whisk into blackberry mixture til it thickens. add remaining 1 cup blackberries. Season sauce with salt and garnish with chopped cilantro.