What causes a Large blood spot on the white part of eye?

A rupture of a blood vessel in the sclera (white part) of the eye will produce a "blood spot". These can occur for various reasons, they are fairly rare, and will likely resolve on their own. Blood spots that do not clear up, tend to reappear or appear in other parts of the eye, should be examined by an opthamologist. A doctor should also examine any injury to the eye caused by blunt or penetrating trauma.