Tempering/Quench lines

[archer]

A while back Alex mentioned a blanched area on a blade caused by quenching. What causes black or darkened quench lines? While we're at it note that the third blade shows a V chevron pointing towards the blade tip. The two upper blades show the first quench line dark and pointing toward the hilts then each show two more quenches with the V changing directions. What is that all about besides I have to much time on my hands. lighting was bad but i think you'll see what I mean.

Thanks, Steve