As stated, blues are very sensitive fish with weak immune systems.
It is possible that he might have new tank syndrome, was your tank developed with enough beneficial bacteria? Also provided hiding places is a plus.
He could also be sick with parasites if he is not eating and if you see any signs of white poop/constipation.
Everytime I get a blue, I treat them with diluted epsom salt via pipette down the throat to flush out the system. Some treat with metro, but I prefer to stick with Natural route first as medication can cause more stress to their already weak immune and digestive systems.
I then treat the tank with probiotics to ensure disease prevention and since I have followed this steps, I have not lost a blue to any parasites/diseases.
Diet is also an important factor. I only feed flakes for the first few inches as they are easy to digest, also no bloodworms as that can cause SBD from my experience.
Here is my current blue with gold gene.
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