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Welcome to BYC. Is it at the hock or elbow? is the hock swollen? Can the chick stand and walk? Sometimes a hock can be swollen and red from a slipped tendon or bone deformity causing pressure on the tendon, but mycoplasma synoviae infection in the joint can sometimes be a cause. Can you try and post a picture or two?
What is the yellow substance around the blister? Mycoplasma synoviae can cause yellow exudate from a swollen joint, and it is a type of bacterial arthritis. Is the blister soft or hard? Hopefully that is not what is wrong, but antibiotics such as Tylan and Oxytetracycline can be used to treat symptoms. Chickens with MS will remain carriers for life. Here is some reading about MS:
hock joint infected with MS
showing yellow exudate