Interesting. I’ve seen them vary in a lot of colors through out the year, likely just a small variation/ phase. I think that color variation is determined by where they’re from, ie: Lake St Clair, Detroit river, Eastern end of the lake, etc. A true “blue walleye” or “blue pike” would be pretty rare since they’ve been extinct for a while lol. Most interesting one I’ve seen was on my ole man caught a few years back on Erie, might’ve been with @Gern186
I'm no expert and maybe Dustin can help me on this but it looks like a fish...
On a serious note I would say it's just camouflage color for that fish. Deep water fish are darker then shallow fish. Clear water versus muddy water. Muddy bottom versus rocky bottom. Fish come in all kinds of different shades and patterns depending where they spend most of their time.
I did a little research and talked to some fisherman, and it seems that it is a recessive gene thing. I also found out that they "Blue Pike" was actually a walleye?