What is the difference between blue blood and red blood?

The term "blue blood" is a phrase used informally to refer to members of aristocracy or high social status. It does not have a literal meaning related to actual blood color.

On the other hand, the term "red blood" refers to the actual blood that flows in the veins and arteries of humans and other animals. It contains red blood cells that carry oxygen throughout the body. The redness of the blood comes from the presence of hemoglobin, an iron-containing protein in red blood cells that binds oxygen molecules.

To summarize, "blue blood" is a figurative expression, while "red blood" refers to the actual blood in living beings.