Colloseum, dark tourist site in Rome
OPAQUE – you visit sites at which evidence of death has been assembled – museums like the Smithsonian. By definition, you're just about a dark tourist, but then so are most people.
DARK – you travel to re-enactments of death or destruction – battle re-enactments. It's a minority interest, but it definitely makes you dark.
DIE-HARD DARK – you go to memorials and interment sites – monuments to death and graveyards like Rwanda's Gisozi Genocide Memorial. You take your dark seriously.
PITCH BLACK – you visit actual sites of death, but a respectful amount of time after it has happened – places like Auschwitz and the Killing Fields. This is the darkest level that is acceptable.
TOO DARK – some people travel to watch death – public hangings, whippings and executions. Likewise, some people travel to sites that still haven't recovered from catastrophe and are not yet ready for travellers. Neither of these have any justification.
Reference: Lonely Planet Blue List, The best in travel 2007