Humans have red blood due to it’s iron based hemoglobin. While Mermaid’s have blue blood due to copper based hemocyanin, with extra paranormal properties, that flows through their supernatural arteries. Both humans, and mermaids transport oxygen through their blood with the lungs providing the source of said oxygen. However Mermaids have special lungs with gill like properties that extract oxygen from water rather than air as is the case in humans. When a Mermaid transmutes to a land based human form their lungs are able to breath air until they enter the sea, and return to Mermaid form. It is actually possible for a human form Mermaid to drown in fresh water but the death would be fleeting. Their very immortal supernatural nature would cause them to revert back to Mermaid form, and in essence self-resurrect.
In the case of human-mermaid hybrids, known as Demi-Mermaids, the dual respiratory role exists in one permanent simultaneous state. In other words a demimermaid can breath both air, and water, to a far lesser extent. Since they are a mix of blue, and red blood they have violet colored blood made up of unique hemoglobanin. An iron-copper alloy based metalloprotein oxygen transport mechanism that could only be found in supernatural beings such as Demi-Mermaids.
Another interesting property of Mermaid blood is the noticeable glow it gives off at night in sea water. Naturally this is something only seen if one is injured. The luminescence is an unknown property of their supernatural natures.