Human blood has been experimentally determined to freeze at -0.52 degrees Celcius, or just barely below the freezing point of water. This makes sense because blood is aqueous (its base ingredient is water) and is mixed with many other things such as cells, platelets, proteins. Generally mixing something into a solution depresses - or slightly lowers - the freezing point of that solution, so one would expect blood to have a freezing point just under that of water.
