Yeah, a black tag is someone who is going to be dead. That may be because they're already dead, or because they're beyond help (given the resources available).

The criteria used by all the major triage systems for a black tag classifications is a lack of respirations (and because he's an ER doc, he's going to reflexively check a pulse).

I don't think my experience is especially important (as you said, this is all googleable info), but I'm a paramedic with a little over a decade in EMS.