Trevor Devall voiced Iron Will in Putting your hoof Down. Fluttershy attends an assertiveness seminar/workshop. Iron Will promises he can turn help bring out any pony's assertiveness or they don't have to pay for his seminar. Fluttershy takes her assertiveness too far when she lashes out at the Mail pony, a tourist, and later on at Sugar Cube Corner, some ponies in line who cut in front of her. When she lashes out at Rarity and Pinkie Pie, she feels really bad.
Well, I think the fact that the artist drew him black might stem more from Iron Will's appearance and character being loosely based off Mr. T, with him going as far to parody one of his catch phrases with "I pity the foal" ^ ^