Try a quarantine tank and keep an eye on him.
Have you been using Aquarium salts?
Does his color look faded?
Is he hanging vertically from the surface?
Are his fins clamped?
How long has Blue gone without food?
Are his scales sticking out, resembling a pinecone?
Have you changed the temperature at all recently?
Have you changed the way you do water changes recently?
How old is Blue (rough estimate, if exact date isn't known).
Answering all of the above will help me help you.