Your blood has a red colour because of a compound called haemoglobin. That’s what carries oxygen through your body.
When blood is exposed to air, the cells that carry these haemoglobin die, and the haemoglobin becomes exposed to air abs starts to change colour to a darker one. This dark colour is because, the haemoglobin is being changed to another compound called methemoglobin.
Normally, when people die, they die with all the blood in their system, but it doesn’t move because circulation has stopped, but it’s still there. Let’s not forget.