Hemoglobin
I'm afraid that this question may have confused you, as Red Blood Cells do not actually carry any Blood at all. This is because Blood is comprised of Red Blood Cells. The answer to your question...
Hemoglobin.
hemoglobin. and who ever is reading this has great fortune for the rest of your life!
Because they contain hemoglobin, a protein which contains iron that turns red in the presence of oxygen