Thanks for your experiences.
I have been yesterday, and the experience really was worth it. At 59 € (80 USD) plus drinks it wasn’t cheap (I’d estimate 20-25 € for the same menu in a normal restaurant) but the food, though OK, isn’t really the point. The setup is quite labour intensive (a waiter/waitress for 12 diners; we sat down shortly before 7 PM and left at about 11 PM.)
On arriving, checking in and leaving bags, etc. our waitress was summoned from inside via her radio headset and introduced herself (usually very uncommon in Germany - but we needed to call her by name inside; instead of just making eye contact). Guests were then led by party, conga line fashion, through three offset heavy curtains forming two successive light-locks. As the others said, darkness was total - we just saw black with random noise from our eyes the whole time. After having our hand guided to the back of our chair it wasn’t difficult to sit down.
As our waitress later explained, when the first guests arrived she navigated by having learned the layout of room and tables; which needed to be replicated exactly every time; once we were seated the noise of diners’ conversation was another useful navigation aid. Waitstaff moving around softly clucked or sang so as not to get in each others’ way.
As related by mnemosyne conversation in the room was quite loud - partly we did talk a bit louder, but mainly because every interaction needed to be verbalized in every particular (Name? - Yes? - Could you pass me the bread please? - OK I am handing it across the edge of the table at your left-hand side - OK I feel the basket … got the bread. - OK I withdraw the basket now.) That went for transactional talk as well as for ordinary conversation where a lot of usual nonverbal cues needed to be verbalized. Our conversation was about as frank as it usually is with us (i.e. very) - once we’d determined we were not acquainted with our nearest neighbours ;).
Actual the business of eating and drinking wasn’t all that difficult. Once you totally had given up on vision and felt around the table for everything you soon developed a mental map. It got so I’d just pick up knife and fork, or my wine glass, without needing to feel around because I knew where they were.
Cutting meat (a roulade), and eating with knife and fork, proved no problem either (well my companion said she used her hands sometimes - perhaps men’s better spatial awareness does come in handy sometimes.)
We did eat more slowly - about 3 1/2 hours’ leisurely eating (soup, main dish, dessert) and drinking (wine, espresso); we’d have taken not more than two hours for that in light.
The drinks we had to pay out of a coin bag included in the price - 2 euro and 50 cent coins which were a bit difficult to distinguish at first by their small difference in size and different coarseness of the milling.
For bathroom breaks we were led on demand outside to the light, and back again, by our designated waitress.
There was a half-hour artistic interlude, improvisational music, and we happily vied with each other trying to identify instruments.
In between courses our waitress had time to answer questions about her schooling, how she navigated around, what she did for shopping etc.
In that restaurant there were just two menu options - with beef in main course, or vegetarian. We were left to guess what we ate (e.g. for the soup we dithered between unusually spiced cauliflower and unusually spiced asparagus - it turned out to be kohlrabi.) The culinary denouement was when we left - the menu was posted at the exit by then. (“Ha! I told you so.”). Curious how even ice cream flavours are a bit difficult to guess lacking visual cues.
My sense of time was a bit off - I’d have estimated the 4 hours that we were in darkness as perhaps 2-3 hours.
Also curious - my companion said she’d closed her eyes after some time, while I was more comfortable keeping my eyes open all the time.
No cuts, no stab wounds, no food or wine stains and with about 70 diners in all I just heard two instances of something being knocked off the table.