From WoWWiki
Of Silvermoon is a title awarded to Horde players upon completing the achievement 
Exalted Champion of Silvermoon, which requires the player attain exalted status with Silvermoon City and earn the right to represent Silvermoon City in the Argent Tournament.
Blood Elf characters can obtain this title by completing 
[80] The Valiant's Challenge (Horde). Characters of other Horde races must complete their version of The Valiant's Challenge, then complete [80] A Valiant of Silvermoon.
Patch Changes
[80] The Valiant's Challenge (Horde). Characters of other Horde races must complete their version of The Valiant's Challenge, then complete 
