Okay, so here's what I'm thinking:
Possibility 1: A second (merle) male got to mom and your puppy has a different dad than the rest of the litter.
Possibility 2: Your puppy isn't actually merle, but some kind of dilute blue. It's very difficult to tell because his colored patches are so small, but I don't see obvious splotches of dark and light color the way you'd expect with a merle. Doesn't mean he isn't one, but I also can't conclusively say he is from these photos.
Possibility 3: Black tri dad is a cryptic merle, meaning he IS merle, but his color genetics just so happen to express in a way that makes it really hard to tell, visually. Thing is, this is pretty darn rare, and if he's merle then statistically half the litter should be as well. Obviously statistics just tells you the most likely scenario, not all the possible outcomes, but it'd still be unusual
Whatever his genetics, he is definitely NOT a double merle - those dogs have a very distinctive 'off' look with pink skin around their eyes and nose/mouth (not always, but often), so I wouldn't worry about color related health issues at all.
There are some dog color genetics groups on Facebook that might be more knowledgeable in what's going on here, particularly regarding the scenario where he's not actually merle. But the only way to know for sure what his genetics are doing is to get a DNA test, either through a vet or a company like Embark - they're most well known for their breed tests, but their full kit also looks at a ton of genetics related to health, coat, color and the like.