Morphine itself has multiple direct effects on the vasculature (145,146) and it was suggested (62,82,147), and is now known to be so, that other haemodynamically-active mediators (148) released along with histamine contribute to the variable response to histamine liberators.
Release of histamine from skin and muscle in the cat by opium alkaloids and other histamine liberators. J Physiol 1951; 114:490-509.