Both of my parents are/were diabetics. For the past few years, therefore, I have taken a keen interest in the relationship between chocolate and the disease. It should be pointed out that I'm no medical professional and this article is just an interpretation of my views on the subject matter.
There may be many reasons to limit your sugar content. Some say it feeds cancer, some say it will make you fat and some will say it rots your teeth. The way I have been brought up is that 'everything should be consumed in moderation'. Your suggested maximum intake of a given foodstuff will be different from everyone else and purely based on your own lifestyle and history.
My mother is diabetic and I take her most of my sample bars after I'm done with her. I leave out anything that is lower than 75%. Every evening she will treat herself to a square or two as part of a balanced diet - with the required consumption of slow release carbohydrates.
I defer my understanding of the matter to the Diabetes UK charity which I believe have claimed that dark chocolate is safe to consume in small quantities as long as your blood sugar level isn't spiked beyond your recommended limits:
We often get customers ask questions about specific bars of chocolate, most notably their sugar content. Where possible we will pass on the information if we are able to obtain it. But here is the information for a number of our most popular chocolate bars (assuming accuracy from our suppliers):
Virtually all the companies listed above have reasonable funds available to test their products, however, some very small scale producers just don't have that capability. What you may be able to deduce, however, is that the higher the cocoa content of the chocolate, generally the lower the sugar content of the chocolate). It should be said, though, that all chocolate, even 100% has some inherent sugars present from the cocoa bean itself. This is minimal, but not trace.
So when choosing chocolate when you have diabetes or you are buying chocolate for a diabetic try and go as high a cocoa content as you can or you think they will enjoy, consume in moderation and watch your blood sugar levels. This may sound extreme, but make sure you have talked through your diet in general, and chocolate in particular with your GP and diabetic NHS (or private medical) professional. I have seen one particular diabetic with extremely high sugar levels consume copious amounts of ice cream and be forced to take daily injections as a result of not taking their condition with the utmost seriousness. So please don't assume that the odd small square will have no impact, it most likely will. But, by the same token, just because you have diabetes it doesn't mean you must exclude all treats totally.