Ordinarily, it's not hard to know when your food has gone bad. If it looks nasty and smells nasty, it probably is. Things aren't that straightforward in the case of blue cheese, though, because it's supposed to be covered in mold and smell like dirty feet. With blue cheese, you'll need to look past its naturally funky nature for other signs of spoilage.
Not All Molds Are Created Equal
Ordinarily, when a cheese gets moldy you'd simply throw it out, but that rule doesn't apply to all cheeses. Conventional cheeses like cheddar, mozzarella or Monterey Jack are made and cured without the use of any mold at all, though they'll usually contain some bacterial cultures to add flavor. For those cheeses, any kind of mold is an invader and a sign that things are heading south.
Yet there's a whole other tradition of cheesemaking; several styles of cheese deliberately use mold in their production. Blue, or Bleu, cheese in its various forms is the obvious example, and so are softer cheeses like brie and Camembert. These use very specific strains of blue, green and white molds that are well-proved to be safe for human consumption.
Will Eating a Little Mold Hurt You?
Clearly not all molds are harmful, because blue and brie cheeses wouldn't exist if they were. Others very definitely are harmful, and create toxins that can make you ill. The USDA has a whole range of guidelines on the subject, but they boil down to just a few simple rules of thumb.
Soft cheeses like queso blanco, cream cheese or fresh goat cheese should just be discarded if they get moldy. With hard cheeses like cheddar, you can cut 1 inch below the moldy area, discard that and still use the rest. With blue cheese, you need to take a long, hard look at the mold you see to decide whether it's the right or the wrong kind.
Mold on Blue Cheese
The mold that's used to create your blue cheese will have a distinctive color, either blue or green or something in between. They're usually pretty good at out-competing other molds, but sometimes you may notice that a different green or blue mold has started to grow. These are invaders and they're a sign you should probably discard the cheese.
Molds in most other colors are also bad news, especially the ones that show up in shades of black, grey, yellow, brown or even pink. The exception is white mold, which is widely used in making cheeses and dry-cured sausages. If you get white mold on blue cheese it just means that your last piece of brie or Camembert left some spores behind and they've taken up residence on your blue cheese. It's not harmful, though it can begin to change the flavor and texture of the cheese. You can eat it as is or cut off and discard the affected area.
Look for Other Signs
Mold isn't the only, or even always the best, way to tell if your cheese is getting past its prime. You may notice that it's beginning to form a layer of slime on its surface or that moisture is beginning to puddle inside the wrapper. Those are clear signs of unwanted bacterial activity, and they mean you should discard the cheese.
Another warning sign is a change in the smell of the cheese. Each cheese has its own aroma. Some of them are pretty strong and funky by design, but they're distinctive. A sharp smell of ammonia is a sign that your cheese has aged past the point of being palatable. You can still eat it, but it's nasty. Any kind of funk that wasn't part of the cheese's original smell is also a bad sign, especially if it's combined with slime and moisture on the cheese or a yeasty smell of fermentation.