Originally, "Realtime Blackhole List" referred to the first blacklist which began operating on the Internet in 1997.  That list is now known as the RBL.  Today a realtime blacklist refers to a list that is updated on a regular basis and the changes are propagated immediately -- in real time.  Almost all blacklists currently in operation may be considered realtime blacklists in this sense, although some lists are updated on a set schedule such as daily.