(For those unfamiliar with the Kastle-Meyer test, a good overview is available here on ScienceLine ).

The indication of a positive test is an intense pink color, which signifies that the reduced form phenolphthalein has been oxidized (i.e., has lost electrons). The important reaction in the test is actually that of hydrogen peroxide (H₂O₂) with hemoglobin in blood. As described here and here, the phenolphthalein is not directly involved in this reaction, but rather just supplies electrons. The hemoglobin turns the peroxide molecules into water by donating electrons. The hemoglobin then wants more electrons to be stable, and they are donated by the phenolphthalein that is also in the solution.

I do not know quantum chromodynamics and so can't describe it in the physical sense, but two electrons come from phenolphthalein , which join with two protons and one hydrogen peroxide (H₂O₂) to create two molecules of water. The protons have to come from water, which means that the reaction creates hydroxide ions.