The best way to tell and know when blue cheese has gone bad is to smell the blue cheese.
When blue cheese goes bad the blue cheese takes on an ammonia like smell so if you smell ammonia like smell from the blue cheese then throw it out as it's gone bad.
Although blue cheese can look moldy and still be good it's not good if the mold look has increased on the blue cheese.
If the blue cheese looks overly moldy and not like normal blue cheese or it smells bad then throw it out.
When kept refrigerated the blue cheese will last in the refrigerator for up to 7 days.
After 7 days in the refrigerator the blue cheese is likely bad and if it smells bad or tastes awful then throw it out to avoid getting sick.