The leucomalachite green presumptive test for blood (also known as MacPhail's 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 leucomalachite green is oxidized to malachite green (a triphenylmethane dye that resembles crystal violet).
The test is a presumptive one because false positives are possible. Like peroxidases from plant origin or chemical oxidants.