The claim states that fresh blood darkens in color upon exposure to air due to the oxygenation of hemoglobin. This is a basic piece of textbook physiological and biochemical knowledge that is universally accepted through everyday and clinical observation. None of the retrieved papers specifically test or focus on the visual darkening of blood upon air exposure as a novel hypothesis, as it is foundational common knowledge. Therefore, following the instructions for STEP 0, this falls under COMMON_KNOWLEDGE and requires no citation.