My understanding (and this is from a limited amount of Googling, so if you have domain experience I'll gladly defer to your judgement) is that black tags are applied to those who are already thought to be deceased, _or_ to those who are thought cannot be saved with the given care available:

"Black tags - are used for the deceased and for those whose injuries are so extensive that they will not be able to survive given the care that is available."

https://www.medicinenet.com/script/main/art.asp?articlekey=7...

Again, if you are experienced and knowledgeable in triage please disregard my understanding.