Thats not uncommon! I've caught channel catfish that were very white, other channel catfish that had many spots and some hardly any, others that were blue in color, and yes even channel cats that are very blackish colored! There are all different types of these and it can just vary based on oxygen in the water or lake, different food they are feeding on, and even different minerals in the water. I've even caught different colored channel cats in the same river!
Its actually all pretty normal. Some have told me it varies based on spawning season also but I'm not sure on that. The bottom line is many things can cause the fish color to vary greatly and its all very normal.
Just like I've caught some sunfish that were really brightly colored in different areas, and the same type of sunfish also was dull with very few colors. No different there, the colors can vary alot.