Previously the same color as the flesh, but these past days it scratched it open with a little blood; what kind of bump is this?

Hello. From the photo, the cat has developed a small abscess. The cause is unclear, possibly tissue effusion or wound infection.

Put a recovery collar on the cat to prevent further scratching and infection. Take it to a veterinary hospital to remove the primary cause and treat accordingly; closure therapy may be used to drain the abscess and apply medication.