A word or two of caution: Methylene blue is an MAO inhibitor. If you are high serotonin it will make it worse. If you are thiamine deficient, you can't clear serotonin out of the brain normally. And, finally, whatever is wrong with you, it is not caused by a methylene blue deficiency.
My understanding is it's pretty difficult to experience serotonin syndrome from taking methylene blue. But if someone is taking medication to increase seratonin, then of course, caution is warranted.
Regarding a "methylene blue deficiency". Not really sure what you're getting at. What I do know is that Methylene Blue is well known to increase oxygen levels in the body, and is used in conventional medicine to increase oxygen levels for all kinds of hypoxic states. If a person is admitted to an ER with carbon monoxide poisoning, it's highly likely they'll be given Methylene Blue to counteract it.
Methylene Blue is also well documented to improve mitochondrial function. Given brain (and heart) cells have the highest concentrations of cellular mitochondria, it makes sense that the brain, along with getting extra oxygen, will also have extra energy. So if somebody who is low on oxygen, and/or low on mitochondrial energy takes Methylene Blue to increase oxygen and energy levels, does that mean they suffer from a "methylene blue deficiency"? I don't necessarily think so. But it would seem their dealing with deficiencies that methylene blue can effectively address.
BTW, MB is also a great anti-fungal, antibacterial, and antiviral agent. Sometimes these kinds of (often undetected) infections can cause all sorts of physical problems, including various kinds of brain dysfunction. Why not take a low-cost, highly effective supplement that can do so many things for the body and brain, especially since there are testimonials online of people with autism benefitting from taking it? Makes sense to me. And it works quite well for me.