My first thought was you were speaking of case hardening. While it may look OK it is no good for a knife!
https://en.wikipedia.org/wiki/Case-hardening
I think what you are speaking of is something like this!
http://www.instructables.com/id/How-to-colour-steel-with-HEAT/
From the link above they are using a pen torch which I doubt that would get hot enough to ruin a HT. Grant you though I have not read through the entire article, nor have I done it myself. Some others may have more info to contribute!!
There are other processes, that I have heard of might be too hot to not hurt the HT on the knife!! Hope that helps.[/QUOTE
I read the article but I don't think this will work third paragraph down reads...
-metal object/s (I used mild steel, Please note this method should NOT be used on tempered/hardened steel as you will effect the properties of the metal.
This is what I thought might be the case?