“Ablaze” enters The Hobbit in its adverbial form (it can also be an adjective), and only in Chapter 14 in reference to Smaug!  The “a-” prefix is the unstressed spelling of “on” as in “on fire” which became one word with use.

• 14.014 seeking only to set their town ablaze.
• 14.017 Soon he would set all the shoreland woods ablaze

“a, prep.1.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.

“ablaze, adj. and adv.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.