One the one hand, the differences between blood blisters and water blisters can be determined by the manner in which they occur and the fluids with which they are filled. On the other hand, the two actually hold more similarities than they do differences. Perhaps the greatest connection between these blisters is that they can be painful, and that they usually appear at the least desirable moment.
A water blister, at least in most cases, is caused by simple friction. Your shoes might be too tight, and the rubbing of leather against toe causes a blister. The same theory applies to almost any repetitive motion where skin meets a foreign object. Too much time spent raking, shoveling, or swinging a baseball bat or tennis racket can lead to a water blister. These blisters can also be a result of sunburn, excessive heat, allergies, or viruses such as chicken pox, shingles, or herpes.
In the case of water blisters caused by friction, the external layer of skin is damaged. However, a water blister is not filled with water. The clear fluid is in fact blood serum, the substance that remains when blood no longer contains clotting agents or red blood cells. The serum collects under the damaged skin in order to both protect the wound and help it heal.
In the case of a blood blister, it is the blood vessels near the skin’s surface that have incurred damage. Injuries of this nature are generally caused by some sort of pinching action. If you should happen to hit the edge of your finger with a hammer, catch it in a car door, or bang it strongly against the edge of a table, there is a good chance you will soon notice a blood blister.
Blood blisters do not occur when the outer layer of skin has been cut. It is the vessels underneath the skin that are damaged, and since the blood from the burst vessels cannot escape, they form into a small pool or bubble. Blood blisters are minor injuries, assuming they are cared for properly and infection is not allowed to set in.
You should never pop a blood blister, or attempt to tear off the skin that covers it. Nor should any attempt be made to drain it. Simply wash the area around the blister, apply an antibiotic ointment, cover with a bandage, and allow it to heal on its own. A water blister can be pierced with a sterilized needle, to drain the fluid, and then covered with an antibiotic ointment and a bandage.