Plus I assume, like better labs and suppliers, you show the customer the print under proper (OK ideal) illumination just as you viewed them when producing the final print. Customer will look at print in shop and either accept or not accept it. If they take that print home and complain due to the 'crappy' lighting, you can tell them the obvious cause. The print didn't magically get darker on the ride home. Client has to be aware and take responsibility for how they view the print outside your shop.
In the old days, all the photo labs I used for printing and processing had very good print and transparency viewers. I might take my rolls of transparency to a client who might examine them by holding them up to a window. OK, they don't really know how to view film. If they accept the film, I'm done. If they complain the image has a cool cast, it was my responsibility to have them view the chrome properly. I don't recall any clients objecting to this very useful lesson in viewing transparencies. Yet the problems with 'my prints are too dark' are largely caused by the same misunderstanding (non awareness if you will) that how you calibrate a display, for a visual match to your prints requires the print viewing conditions be sound. This isn't rocket science. It's actually just common sense. Again, the finest print you ever viewed will look like crap under crappy lighting. Don't use crappy lighting! I'm aware that this isn't always possible. But altering the good document data or expecting some ICC profile setting will fix this doesn't wash.
Of course, some of the imaging guru's would rather provide a more complex "solution" since tips to alter the data in the applications they write about is where the money is. Make a perfectly edited image lighter using this or that technique because the print is viewed in a dim environment. Makes about as much sense as always pushing your film 1 stop in processing because you always under expose your capture 1 stop. KISS!
I fear we are starting to wax nostalgic, or perhaps have been doing so already. However, I must reply to your comment about pushing film, most notably transparency film. In the mid 90s I put my business into a state of hibernation and managed another lab here in town for almost 5 years. One of my first duties was to try to bring the E6 process back in line, which was not an easy task. The last transparency film process I had been involved with was back in the early 70s and was then called E3, mostly 8x10 and 11x14 film stock for catalog houses. Obviously I had to start all over again to educate myself in the E6 process. I soon discovered that none of the staff had ever plotted curves to get a grasp of what might actually be happening in all of those chemicals, so that was first on the agenda. One day I asked the fellow who was in charge of that dept(loosely put) how he had been handling quality control for the transparency process. His answer was to tell the customers to underexpose their trannies by about 1.5 stops, since the process seemed to be slightly "hot". Remember, he said "Slightly Hot". My first curve showed that his idea of "Slightly" was actually very close to 2 stops overcooked. After about two weeks of intensive work I finally brought the process back to where it should be and we went from there to become a completely qualified Kodak Q-Lab for E6 processing. A very in depth procedure of chemical measurements and adjustments which was capable of keeping the process online consistently. It also included sending reports to Kodak to verify the status of the process and offer suggestions, should it begin to stray. Again, standards to be met, both for the lab and the customer. Our customer base for E6 grew quite nicely, including a couple of large consumer related accounts. All of this to point out once again the absolute necessity to aim for Quality Control in every area of the business. Today in the digital world it's called Colour Managemnt, which is simply another way of saying the same thing. Yesterday's QC is today's ICC. It's no longer good enough to tell the customer what the print etc should look like. No, that conversation should never have to take place because there should be NO doubt in the customer's mind that it is indeed exactly as it should be. And of course the viewing environment in the lab should always be in line with the accepted standards as well. If the customer then chooses to display the print in less than ideal lighting(AKA, Crappy Lighting), that should also be pointed out and rectified if possible.
Well OK, so much for my latest rant. However, I do believe that you Andrew will certainly be able to identify with much that I have written here. I'll leave it alone for now and let everyone else get on with solving the mystery of the "Dark Print". Or, as Guy Noir would perhaps say...It was a grey night on the fifth floor of the Acme Building. The person on the other end of the line spoke few words, "My prints are too dark". That set in motion the latest case of Guy Noir, Private I/Eye.