Histamine is created and released by the body, often as an immune response. When you feel the flush its the body releasing histamine from white blood and mast cells. Histamine increases the permeability of the capillaries to white blood cells and some proteins so by taking niacin and causing the release of histamine you are actually empowering your immune system.
Dr's Hoffer, Saul and Foster have done significant things with niacin, much of it done back in the 50's and 60's when one could easily recommend supplementation and for quite some time had a captive audience in various mental health wards where niacin proved to be hugely beneficial for mental health....as are most B vits.
In their book "Niacin the real story" they have 20 pages of reference material as well.