ANSWER ATTACK:
It's simple. Roy dips his sword into a can of baked beans before each match. He then sharpens it on a cats tounge, and juggles watermelon while complaining about the price of nutmeg. After the cat is well and truely dead, and Roy's sword is coated in beans and blood, he'll go have a shower.
*SCENE DELETED*
.. And that's why it flames when he hits someone.