Preheat oven to 425°F Coat a baking sheet with cooking spray.
Mash black beans and cumin with a fork in a large bowl until no whole beans remain. Stir in corn and 1/4 cup breadcrumbs. Combine tomatoes, scallions, cilantro, 1/2 teaspoon chili powder and salt in a medium bowl. Stir 1 cup of the tomato mixture into the black bean mixture.
Mix the remaining 1/3 cup breadcrumbs, oil and the remaining 1/2 teaspoon chili powder in a small bowl until the breadcrumbs are coated with oil. Divide the bean mixture into 8 scant 1/2-cup balls. Lightly press each bean ball into the breadcrumb mixture, turning to coat. Place on the prepared baking sheet.
Bake the croquettes until heated through and the breadcrumbs are golden brown, about 20 minutes. Stir avocado into the remaining tomato mixture. Serve the salsa with the croquettes.