Final answer:
The Marquis reagent is not used in testing for blood. Instead, it is primarily employed in the identification of alkaloids and certain other compounds by changing colors when reacting with different substances.
Step-by-step explanation:
The method not employed in testing for blood among the options provided is the Marquis reagent. The Kastle-Meyer test is a presumptive blood test where the chemical indicator phenolphthalein is used to detect the possible presence of hemoglobin. The Luminol test involves the use of luminol, a chemical that exhibits chemiluminescence, with a blue glow, when it comes in contact with blood. Microcrystalline tests are microscopic techniques used to identify types of hemoglobin in blood residues.
However, the Marquis reagent is not a test for blood. Instead, it is a reagent used primarily in the presumptive identification of alkaloids as well as other compounds. When used for its intended purposes, it turns different colors when coming into contact with different substances.