Phenolphthalein is a presumptive test that reacts with the heme molecule present in blood. A positive reaction gives a pink color. While bloodstains normally appear red-brown in color, the color of the substrate or the age of a stain may affect the appearance or visibility of the stain.  Phenolphthalein tests are typically conducted on suspected bloodstains prior to collection.  While a positive phenolphthalein reaction is indicative of blood, it is only a presumptive test and false positives are possible.  Additionally, the reaction is not species specific.  Positive reactions are not limited to human blood.

The swab in the image below shows the characteristic color of a positive reaction with the phenolphthalein test, indicating the presence of blood.