Sadly, it’s not as simple as that. My dad is a practical joker and was a pharmacist when I was a kid; he brought home methylene blue to experiment with. Unfortunately it’s strongly alkaline, so 1) it has a very bitter flavor that’s hard to mask and 2) it makes carbonated beverages fizz violently and then go flat.
You might get away with slipping it into somebody’s coffee - assuming that they don’t actually like coffee but drink it out of some sort of compulsion and will choke down any sort of bitter sludge - but I can’t think of any other way to sneak it into a drink. (Yes, I saw that episode of MAS*H too, but it was a TV show.)
In case anybody’s ever wondered what the actual medical purpose of the stuff is, by the way - it’s used as an antidote to cyanide poisoning. Cyanide does lots of nasty things to the body; one of them is binding so tightly to hemoglobin that it can’t transport oxygen. Turns out that cyanide likes to bind to methylene blue even better than to hemoglobin, though - so if your friend is turning blue, give them blue dye and they’ll soon be in the pink again! They WILL pee blue, though.
