The phrase "black suit" refers to clothing made from black fabric, typically consisting of a jacket and pants or a dress for formal occasions. Full definition
Suddenly she noticed several men in black suits and sunglasses «all looking super professional» along with what she described as a herd of golf carts. (sportstarsmag.com)
Try pairing a simple black suit with a crisp white shirt. (theidleman.com)
He used an old black suit jacket and pants for his costume and, the morning of Historical Figure Day, he used a black marker to color in a beard on his face. (educationworld.com)