Preheat broiler with oven rack in top position. Arrange jalapeños on a foil-lined baking sheet. Broil, turning occasionally, until charred on all sides, about 8 minutes. Wrap jalapeños in foil from pan; let stand 10 minutes. Rub off skins; discard skins. Discard stems, membranes, and seeds. Finely chop jalapeños. Reduce oven temperature to 350°F. Process beans, cream cheese, mayonnaise, cumin, salt, and garlic in a food processor until smooth, stopping to scrape down sides of bowl as necessary. Add jalapeños and onions; pulse just until combined. Spread mixture evenly in a 9-inch pie pan coated with cooking spray; top evenly with Colby Jack cheese. Bake at 350°F until cheese melts and dip is bubbly, about 20 minutes. Remove dip from oven; sprinkle with bacon and cilantro, if desired.