Thank you for the opportunity to help you with your question!
Hi, it's actually a misconception that blood is blue. A lot of people think that our veins are blue because the blood inside is blue due to being deoxygenated. This isn't true however. Blood is always red. When blood is oxygenated such as in an artery it is a bright red. When it's deoxygenated such as in the veins it's still red, just a little darker. Blood is red because it contains hemoglobin which contains iron that binds to oxygen. Your veins just appear to be blue due to the way the light hits the tissues.
Please let me know if you need any clarification. I'm always happy to answer your questions.