There is a common misconception that blue flames are always hotter. The two pages below explain why this is not always true.

"Blue flames are hotter?"

http://van.hep.uiuc.edu/van/qa/section/States_of_Matter_and_Energy/Fire/20030117075106.htm

"Blue flame"

http://van.hep.uiuc.edu/van/qa/section/States_of_Matter_and_Energy/Fire/20020116210721.htm