Yup. Carbon dioxide (the waste product of aerobic respiration) does change the pH of your blood (carbon dioxide forms carbonic acid in the presence of water). When the carbon dioxide level goes up in your blood, the pH goes down. In fact, your body looks for that change in pH and uses it to control the amount of oxygen which it sends to your muscles. When the pH of your blood goes down, your body is producing a lot of carbon dioxide. This also means your body is using a lot of oxygen. Therefore, your body needs to get more oxygen to your tissues. Your hemoglobin (which transports the oxygen to your body) responds by releasing its oxygen more easily when the pH of the blood is lower. Do you see the relationship?

Lower blood pH means higher blood CO₂, which means your body needs more oxygen. Consequently, your hemoglobin doesn't bind as tightly to the oxygen, making it easier for your muscles to get (FYI: this is known as the Bohr shift).

You made a great connection between what you saw in water and what happens in blood.You probably know that blood is mostly water, so it makes sense that the pH would go down in the watery part of blood. Blood also has special molecules inside red blood cells called "hemoglobin". Hemoglobin is very good at carrying oxygen and carbon dioxide. This means that even more carbon dioxide can "fit" into blood than can "fit" into water, so the pH can change even more. In fact, low pH is one way your body senses it has too much carbon dioxide and not enough oxygen.

Carbon monoxide is found in car exhaust and smoke. It sticks to hemoglobin 200 times tighter than oxygen does. Why is it unhealthy to be around too much car exhaust?

I see that you have access to a computer. You can learn more about blood at
http://sln2.fi.edu/biosci/blood/blood.html

It sounds like you did a really neat experiment to look at blood pH and that you have really thought about what might be happening--way to go! You know that when you exhale you expel carbon dioxide as a major product, which is probably what caused you to HYPOTHESIZE that carbon dioxide (CO₂) could change the pH of a solution, such as blood. Well, you are correct, it sure can change the pH. It works in a simple acid-base type reaction. The CO₂ combines with water in the blood to form carbonic acid, which then gets dissociated into bicarbonate and hydrogen ions. An enzyme, called carbonic anhydrase normally catalyzes this reaction in the body. When you blew into the blood, you were putting in extra CO₂, which caused the pH to change.

Can you think of an experiment that would test your hypothesis directly ?
(HINT: what if you could get a small canister of CO₂ for use in your experiment?)
Good luck--keep up the good work!

yes. CO₂ + H₂0 -> H₂CO₃ (carbonic acid) -> H+ + HCO₃-. It is an important buffer (pH stabilizer in your blood). When you hyperventilate, you drive off CO₂ from you blood; what does this do to your blood chemistry?