Blue cheese contains an edible type of mold, which is where it gets its blue color. This makes it difficult to identify when the cheese becomes unsafe to eat. However, there are several different methods to recognize unhealthy mold and bacterial growth on blue cheese. The basic senses of sight, smell, taste and touch can usually safely diagnose the state of blue cheese. If unsure, all of these steps should be performed. If any doubt remains, do not eat the blue cheese. It's better to be safe than sorry.
Check the color of the blue cheese. If the white parts are beginning to turn yellowish, or the blue parts are beginning to turn green, don't eat it. If any part of the cheese is a color other than white or blue, it should be discarded.
Smell the blue cheese. If it smells like ammonia, it is beginning to go bad. The smell of blue cheese can become very potent over time, and this is normal. When it begins to smell like ammonia, it shouldn't be eaten.
Take a small bite of the blue cheese. If it tastes like blue cheese, only stronger, it's safe to eat. If the taste seems "off" in any way, it's better to discard the product.
Look for fuzz and mold that wasn't there when the blue cheese was fresh. Any additional growth could be a new, unhealthy bacteria forming on the cheese.
Check for slime. Blue cheese can develop a slimy texture when it begins to go bad. If there is any excess moisture, or if the texture is no longer dry and crumbly, it should be discarded.
Often, only the outer skin of the blue cheese will become moldy. This skin can be removed and the inner cheese may still be edible. However, make sure you follow the guidelines above before eating any part of questionable blue cheese. Harmful bacteria growth can lead to food poisoning.