Blood becomes more acidic because the CO2 and H20 molecules are converted into bicarbonate ions (H2CO3 and HCO3) + Hydrogen (H+) ions, which are acidic.
Carbon dioxide decreases(more acidic) blood pH by bonding with water to form carbonic acid. This is why hyperventilation causes your blood to become more alkaline. It rids your bloodstream of CO2. This is also why we have evolved to have a slightly acidic blood pH. Wiki carbonic acid.