Finish treatments?

Could be the colour left after tempering. If the blade was polished before tempering or a second tempering was done after the polishing stage the steel would form a colour on the surface. The colour would vary depending on temperature. Blue/purple are very close on the temp scale and both colours would form if tempered around 560degF. The hollow was probably polished again to remove the colour but the tang/tail left.
I recently made my own razor and got yellow, gold, red, blue and purple all on the same blade, although the colour was much deeper than on yours. I could be wrong though and it may be a bluing compound or something else.

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Originally Posted by pixelfixed

Most likely because it is worn.

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This was my thought too, Bluing can take on a completely different look as it ages

If you see a Pre 64 Winchester or an older Weatherby the has been lovingly rubbed with a soft oil rag over the years there is a look that it develops that is near impossible to match right outta the Bluing baths...

So true Glen,another perfect example are the 1886 winchesters,they turn purple over the yrs.Also if you can look at a N.O.S orig colt python,you can see the purple under the blue.If I could get Nitric acid (cannot in Kali) would give it a shot.

well, I'm willing to give it a try! Nitric acid eh? Perhaps a trip through the Brownels catalog is called for here?

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Originally Posted by Magpie

well, I'm willing to give it a try! Nitric acid eh? Perhaps a trip through the Brownels catalog is called for here?

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Actually,Brownels will be of no help,Due to OSHA laws nobody sells evil stuff anymore.
you need Nitric acid,this stuff can kill and maim you.
The process is simple,You boil and degrease the item in water and boraxo.
You apply the nitric,you let it work a few minutes,you rinse it off in hot water,than you card it off with 0000 steelwool that has been rinsed in gasoline to remove the oil in the steelwool,after about ten applications it will be dark grey,after 25 it will be dark brown, after 35 it will be blackish,after 50ish it will be blueish.
all your doing is creating rust in a controlled manner,It really is simple no brainer stuff.
But using evil componants.
First time I did it in my shop,all my machine tools had rust on them,best do it outside.