When was 华/華 first used as a reference to China?
In 春秋左傳 (722/468 BCE). http://ctext.org/chun-qiu-zuo-zhuan has the statement 楚失華夏.

Huaxia used to denote tribes that later became the Han people, and later the term shifted to symbolize China. Hua is the beautiful clothes worn by the Chinese, xia is for the grandness of the culture.


  「中國有禮儀之大，故稱夏；有服章之美，謂之華。」《春秋左傳正義》


References:

Wikipedia: Huaxia

維基百科：華夏

維基百科：華夏族

Wikipedia: Xia Dynasty

維基百科：夏朝

維基百科：中華