I have a small blue hard bump at the palm side of my index finger's middle joint. it's been around for 3 weeks disappeared for 2 days and is back.

Vein thrombosis: The most common cause of this kind of bump is trauma to a blood vessel in the palm resulting in a little clot. It is not dangerous for spreading. You experienced the normal course (disappearance in 2-3 weeks) and it was probably reinjured. However, if it persists, you should see a Dermatologist to make sure it is not a benign growth of blood vessels, pyogenic granuloma or hemangioma. Good wishes:)