The action you're referring to, where chefs in old movies take their fingers and scrape the grill of the stove, leaving behind black residue, is a classic movie trope that is not reflective of real culinary practices. This theatrical gesture is often depicted in movies to add drama and intensity to the scene, creating a sense of urgency or frustration for the character.
In reality, professional chefs would not use their fingers to scrape off grill grates or leave behind black residue on purpose. It's unhygienic and can be unsafe, as the grates can be extremely hot and cause burns.
Professional chefs and cooks do use various tools like brushes, spatulas, and grill scrapers to clean grill grates and remove any leftover food particles or residue. Keeping the cooking surface clean is essential for proper food preparation and preventing flavors from transferring between different dishes.
The portrayal of chefs scraping grill grates with their fingers in old movies is simply a creative embellishment for storytelling and entertainment purposes, rather than an accurate representation of culinary practices.