Methylene blue is an actual antidote for methemoglobinemia which is a blood disorder in which red blood cells become really bad at releasing oxygen. It's usually caused by a reaction to certain drugs like lidocaine or compounds containing nitrites. It looks like methemoglobinemia can be caused by "poppers" (which RFK Jr thinks are the cause of HIV).

Here's an article from the Cleveland Clinic that goes into more detail.

The "poppers" thing is probably why RFK Jr takes methylene blue because he likely thinks its a way to prevent HIV. (For which there is now a pre-exposure prophylaxis shot with a nearly 100% success rate and progress on a vaccine....but it's an mRNA vaccine so RFK is trying to kill it.)