The phenolphthaleine presumptive test for blood (also known as Kastle-Meyer test or Kastle-Meyer  reagent) has been in use for the detection of bloodstains since the beginning of the twentieth century. Like other presumptive tests it relies on the ability of heme in blood to catalytically oxidize compounds in the presence of hydrogen peroxide (or similar like sodium perborate, barium peroxide). In this case the colorless phenolphthalin is oxidized to the well-known acid-base indicator phenolphtaleine which in a basic solution is pink.
The phenolphthalin is prepared by reducing phenolphtaleine with zinc powder.
The test is a presumptive one because false positives are possible. Like peroxidases from plant origin or copper salts can do the oxidation.
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Phenolphthaleine presumptive test set for blood.

Left: phenolphthalin. Middle: after oxidation and removing of a proton by a base the pink phenolphthalein is formed. Right: in an acidic environment a colorless form of phenolphthalein is formed.

See also Wikipedia.