I am reading the Lord of the Rings again, and this quote by Saruman

'Later!' he cried, and his voice rose to a scream. 'Later! Yes, when you also have the Keys of Barad-dûr itself, I suppose; and the crowns of seven kings. and the rods of the Five Wizards, and have purchased yourself a pair of boots many sizes larger than those that you wear now.

caught my attention. A while ago, someone was telling me that there were five wizards: Saruman the White, Gandalf the Grey, Radagast the brown, and two others, the Blue Wizards. It seemed from the following conversation that nothing was known about them.

Who are the other two, the Blue Wizards?