I do not think it becomes brittle or weaker when heated as you see a lot of this done by makers. Colors will range when heated. Be careful when you torch the liner that holds the ball bearing for the ball bearing detent as the heat can cause the hole to expand and let the ball bearing fall out. If it does....no more detent to hold the blade closed.
Also, you might want to finish the liners to a satin finish or mirror if you can, before you torch them as the finished product will be more pleasing to the eye. Oh yeah, do not hold the torch in one spot too long.