Your first reaction to this headline might have been, "What do you mean, is blue cheese gluten-free? Isn't all cheese gluten-free?" Well, the short answer is yes. But blue cheese is a potential corner-case that needs some investigation. The reason is that there are steps in the production process of blue cheese where the potential for cross-contamination of gluten is definitely a possibility.
Most people know that the blue in blue cheese is actually mold—penicillium mold to be exact, which during aging breaks down the fats and the proteins in the cheese to change its texture to a silky smooth, and to add depth and piquancy to its flavor. Originally the mold would have jumped off the walls of the cave in which the cheeses were aging and into the cheeses themselves, but for many hundreds of years cheesemakers have intentionally inoculated the mold into the cheese during manufacture, before aging.
Before the advent of modern microbiology, the only way the cheesemakers would have had access to the mold was by growing it themselves. And that they did, by leaving loaves of bread out in the caves, getting them all nice and moldy, and then tossing that mold powder into their milk. Some of the more traditional blue cheese producers today still follow this practice, which is why there are some who suspect that blue cheese may contain traces of gluten from the bread.
Well, celiac sufferers need worry no more. The Canadian Celiac Association recently released a study showing that blue cheese is in fact gluten-free. The study tested three blues where the mold is harvested off of gluten-containing media during production: Quebec's St. Benoit and Alexis de Portneuf, and Roquefort. They also tested two cheeses where the mold was grown on gluten-free media.
In all cases, they found that the resulting cheeses each contained less than one part per million of gluten. For comparison's sake, the current definition of "gluten-free" being considered by the FDA is 20 parts per million. This puts these well below the limits set by the FDA (as well as its European equivalent). So blue cheese is a go for celiac sufferers! Enjoy!