A sleeveless black dress in summer requires the right fabric to feel appropriate and respectful rather than casual. Look for heavier cotton blends or linen that have structure and weight. The dress should have a defined neckline that feels formal rather than open. Sleeveless in this context means you have a simple armhole without straps or embellishment. Black stays black even in summer heat and it is what the occasion calls for. The length should be knee length or longer to keep the dress formal. A simple sleeveless black dress works because it is honest about what it is without any visual tricks or details that would feel out of place.