If it is a blood clot in the vein, it can be quite dangerous.. I am taking this from the “human” perspective of having a blood clot on the vein. Its my suggestion to NOT rub it out. Blood clots will break up on their own – if given the proper medications like a blood thinner. If a clot comes loose it will travel the body and can end up in 3 places: The heart, causing a heart attack; The brain, causing stroke; and the lungs, causing pulmonary embolism. Blood clots are very deadly unless treated. Up to 200,000 people die every year from a blood clot in the vein coming loose. It is not deadly in all cases.. but in many.. and it is suggested to seek help immediately.
Also, I doubt that it is an air bubble in the vein. Bunny would probably be dead it if was. This is called an embolism. Even plants who have embolisms (as often happens) that vein will die. Fortunately plants have many veins that go to the same places.. and maybe only a leaf will die. People (and bunnies, animals.. etc) are not the same.. all veins are interconnected. If there is an embolism it causes the blood flow to basically stop, because no “nutrients” (minerals, oxygen..) can progress beyond the point of the air.
Anyway. Im not a doctor yet.. this is just what we learned in class. So go to the doctor (vet) to find out what is going on.
I am not meaning to scare you. You bun may have somthing else and it not be a blood clot. But if it is, I ASSUME that the mechanism would be the same for a rabbit and a human. And it would be my suggestion to go to the vet. However, I do not know if what I said is true for bunnies, I only know that it IS true for humans.