Dreaming of blood can have one of two meanings.
It can signify that we feel that a sacrifice is being made on some level.
This links into the ancient belief that the blood somehow contained the life of the spirit and, therefore, spilt blood was sacred.
It can also represent renewal of life through its connection with menstruation. Many people fear blood and thus a dream about blood can highlight the need to come to terms with these fears. On a more spiritual level it represents the blood of christ.
Also consult the individual entry for blood.