As you discover star stones in your journey, they will activate (becoming inert stones) and a new portal will open. Bloodstones however must first be used, unlike star stones when merely approaching one would activate it. Bloodstones remain as bloodstones even in your inventory until you use it.
Hint: I'd hoard some bloodstones if I were you. They are incredibly useful and there are a couple of points in the story where you have the option of using them on NPCs.