If you ever catch a catfish and think it is a blue here’s about the only way you will able to distinguish the difference. A channel cat has a rounded anal fin with 24-29 rays. A blue has a long, straight-edged anal fin (tapered like a barber’s comb) with 30-35 anal rays. If you catch a smaller catfish and it has spots on it that is a channel because a blue will never have spots.