Blue cheese is probably the most famous of the various cheese styles. Distinguished by its veins of blue or green mold, but sometimes they also appear like parsley florets. The blue mold is usually Penicillium Roqueforti, but might also be Penicillium Glaucum, which you can find in the pasteurized cheese Gorgonzola. Blue cheeses are made from most types of milk, the most common being cow and ewe. Famous varieties are Roquefort (ewe), Stichelton (cow) and Bleu d’Auvergne (cow), but there are many others.
Some of the French blue cheeses bear names such as Persille(s) de.. As mentioned above that is because sometimes the blue mold appears like parsley florets. Quite obvious, no rocket science.
Normal procedure during cheese making is that the Penicillium culture is added to the milk while in the vat. After the cheese has drained, transferred to the forms and drained further, it is usually penetrated by needles so the mold can get air to breath and subsequently grow. Blue cheese is matured from a few weeks to a year, depending on style.