>>723664021
Mario's eyes were black/brown in his earliest pre-SMB1 incarnation artwork, when he was still Popeyemaxxing. They just made them blue when the art style became more standardized come SMB1 because it's easier to add visual definition between the iris and pupil when they're not both dark colors and it stuck, much like his overalls, gloves, hat, distinct nose and mustache. And the big expressive eyes help them be more endearing compared to characters like Wario and Waluigi who have small beady eyes.