Answer: A little more that 5 Kg for a healthy person
Explanation: First, we know the following:
The regular adult has from 9 to 12 pints of blood. This is around 5 liters for a healthy male adult.
The human body is composed mostly on water, around 80%.
Blood is mostly composed on plasma, which makes blood thicker than water.
Knowing that, almost all the body is compose of water, it is safe to think that blood density should be near to that of water but higher.
The density on water is a know value. Which makes the following true:
1 Liter of Water weights 1 Kg
It could be said then, that the total mass of blood for a healthy person should be a little more that 5 kgs.