Blue cheese is made with milk from cows, sheep, or goats and ripened with cultures of the mold Penicillium. As blue cheese ages, cheesemakers spike the cheese with thin rods in a process called "needling", which lets oxygen into the cheese round and allows the Penicillium to grow, creating veins of mold throughout the cheese that produce the sharp and tangy flavor blue cheese is known for.
Blue cheese is quite safe to eat. The Penicillium mold used in blue cheese does not produce any of the mold toxins that are dangerous to human beings. On the contrary, Penicillium is quite healthy and has natural antibacterial and anti-inflammatory properties.