It's all about combs and wattles at this early age. How big they are and how colorful they are. Those tails will molt out and really won't tell you anything until the birds are much older.
It is a cockerel. Male specific feathering is just starting to peep through in a couple of spots. You can see a bit on the shoulder and the saddle-- it is the brighter colored feathers coming in.
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WTBs should not have feathered legs. The blue chick is probably an olive egger or Marans.
The second chick looks to be an olive egger. I say that because I think it will be barred, not a characteristic of WTBs.
Substitutions are pretty common for hatcheries.
None are looking like definite males, but the first comb is pink. The fourth comb shows the least development. Everyone else is in between. I don't think it is enough to go on for accurate gendering. It will take more time.
Any dark breed can get the black combs and wattles.
This looks like a female to me at that age. If it is indeed a Mystic Marans, those are sex linked, so you're home free.
I could see it. Usually it is the male specific feathering that shows the leakage, so it may come later. Anyway, that is my guess based on the appearance of the chick.