Slice Swiss chard stems into 1/2-inch pieces. Coarsely shred the leaves. Heat a large skillet over medium heat. Add cumin seeds. Heat until fragrant, about 2 minutes. Add oil to skillet and increase heat to medium-high. Add chard stems and salt. Cook, stirring occasionally, 3 minutes or until stems are crisp-tender. Add sweet pepper, onion, garlic, and chard leaves. Cook 3 minutes or until vegetables are barely tender and leaves wilted, stirring occasionally. Stir in black beans and vinegar; heat through. Remove from heat.
Meanwhile, preheat broiler. Arrange tortillas on a large baking sheet. Divide cheese among tortillas. Broil 5 to 6 inches from the heat for 1 minute or until cheese is melted and bubbly. To serve, spoon chard mixture onto tortillas. Top with cilantro and sour cream. Sprinkle with chili powder, if desired.