Roland Griffiths at John's Hopkins is trying to do proper research on psilocybin, i.e. with control groups. Although it's kind of hard since the effects of psilocybin are rather noticeable.
I forget which interview I was listening to about doing LSD research but I remember the gist of the quote: "We tried doing double-blind studies, but 15 minutes into the experiment we realized it was pointless. The people who were given LSD... they knew they'd been given the real stuff, the scientists knew very obviously who'd been given the real stuff... There's no way to give someone LSD and not have everyone in the room know what's going on."
That's a very interesting problem. I wonder if one solution to the 'control group' is for the control to be an alternative intoxicant (i.e. not another psychedelic).
Yeah, it looks like the study linked in the original article didn't do the measurement on the subjects while they were under the influence, but rather some time after the fact. In my experience, psilocybin can have a lasting psychological effect long after the physiological effects wear off.
Not every medical procedure can or has to undergo double blind testing with control groups.
Pretty much any treatment that has very aggressive effects is usually done without it.
Even treatments that can be tested using a control group are quite often tested without it if you can isolate your metrics well enough.
But technically you can still use a control group for psychoactive drugs because in every trial the first rule is not to give the test subjects any information about possible effects or side effects in order not introduce bias, you also quite often isolate the subjects from each other completely.
I'm also not sure if the dose that is given in this case is even high enough to induce hallucinations in the first place.
I would guess that not telling someone could also affect the outcome. You might have a different experience from the treatment if you're wondering and waiting.