Final answer:
The substance in blood that prompts the color change in the Kastle-Meyer test is hemoglobin, which reduces phenolphthalein to its pink form, indicating the presence of blood.
Step-by-step explanation:
The substance in blood that causes the color change in the Kastle-Meyer test is called hemoglobin. The Kastle-Meyer test is a presumptive blood test in which the chemical indicator phenolphthalein is used.
When the test is administered, the clear phenolphthalein solution turns pink in the presence of hemoglobin due to hemoglobin's ability to facilitate the reduction of the phenolphthalein to its pink form. This test is commonly used in forensic science to detect trace amounts of blood at crime scenes.