1933 paper:
https://journals.physiology.org/doi/abs/10.1152/ajplegacy.19...
"Methylene Blue as an Antidote to CO Poisoning", Matilda Moldenhauer Brooks
Clinically, methylene blue is used to treat a different condition, methemeglobinemia and is not used to treat carbon monoxide poisoning which relies on hyperbaric oxygen therapy.
[1] - https://neocities.org/