The last issue of Der Flaggenkurier (#45), dated May 2017, shows the regional flags of Chile and the flags of the regional capitals. For Rancagua, however, only the arms are shown.
The image show also the gonfalon, which is black (I assume that is from the black flag used in the Battle of Rancagua) with the name (Ilustre Municipalidad de Rancagua), logo, name (RANGAGUA) and motto (Ciudad Heroica) all in white. Jaume Ollé, 2 July 2017