plot no jutsu.
or his lost his real eye in a battle or war.
He probably already had his eye
Itachi killed Shisui an Danzo took one of Shisui's eyes. Itachi put the other in the crow he implanted in Naruto.
Danzo had it covered, because he can't turn it off (like Kakashi) and it'll take up unnecessary chakra. Also, he couldn't be seen with a Sharingan :|
Maybe he always had a thing for jacking sharingan's?