Typically the blue tips are created using a torch. If you have watched the new /Drive videos of the Koenigsegg One:1, they show it being down to their titanium exhaust tip. I am at work so I can't post the video.
I would try cleaning the tips as best you can and then just try burning them again. I have never done this but it seems logical in my head.
I just googled it and found two different methods (the second is like my suggestion):
1.
http://my350z.com/forum/body-interio...-titanium.html
2.
http://www.8thcivic.com/forums/diy-h...ue-purple.html