Both results are due to oxidation, essentially components of blood reacting with oxygen in the air.
The color change is due to the iron ion in hemoglobin, the protein in red blood cells that carries oxygen, getting oxidized to methaemoglobin
. The main color difference comes from the different oxidation state of the iron ion, and how this affects its interactions with light.
The hardening is due to the oxidation of platelet cells in the blood, which are responsible for causing the blood to clot. One way in which platelet cells regulate blood clotting is by controling the amount of negatively-charged phospholipids on their surface. Normally, the negatively-charged phospholipids are facing inside the cell, but to activate clotting, the platelets will start moving more of these negatively-charged phospholipids to the outer-surface of the cell. On the outer surface, groups of these negatively-charged phospholipids can form a platform onto which enzymes like prothrombinase can assemble and begin a series of enzymatic reactions
that activate blood clotting.
One enzyme that regulates the balance of negatively-charged phospholipids between the inside and outside of the cell is the protein scramblase
, which helps transport the negatively-charged phospholipids to the outside of the cell. It turns out that, when exposed to oxygen, the oxygen can react with the protein and increase its activity. Thus, exposure of platelets to air activates scramblase, which leads to the platelets flipping their negatively-charge phospholipids to the outer leaflet of the plasma membrane, which allows activation of the coagulation cascade and blood clotting.