Being a professional bladesmith, Ill try to answer your question with some
helpfull info.
1. when you say " black coating" you need to be more exact and specific,
what type of coating is it ? is it PVD-BC ? is it ELECTRICALLY APPLIED OXIDE ?
is it TiNitride ? is it just plain old "tool black" ?
BECAUSE EACH OF THESE ARE APPLIED FOR DIFFERENT REASONS TO METAL
NONE OF WHICH HAS ANYTHING TO DO WITH MAKING THE BLADE STRONGER.
YOU WILL BE SADLY DISSAPOINTED TO LEARN THAT MOST MODERN KNIFE MAKING COMPANIES USE A COATING TO HIDE FLAWS IN BLADE METAL THAT COMES FROM BUYING "SECONDS" IN QUALITY FROM THE FOUNDRYS. A GOOD
TiNi BLACK COATING CAN/DOES HIDE SEVERE PITTING, CORROSION, BAD SURFACE TOLERANCES... THAT WOULD COST A FORTUNE FOR THE COMPANY TO WORK OUT OF THE BLADE, MOST ACTUALLY WOULD END UP IN THE TRASH. I cant tell you how many hundreds of blades ive removed coatings from to mirror polish the blade and found what looked like moon craters or 00 shotgun blast in the blade, and alot of them have been filled with some metalic epoxy to fill the voids and pits.
As the other Gent said, he puts coatings on tools, why ? because it makes an inferior metal temporarily preform like a superior alloy.. untill it wears off..
( not dissing your job.. just explaining the reason its done)
since you SHOULD ONLY own a blade that is NOT STAINLESS in a katana, the deposition of MOST ANY SURFACE COATING will cause the blade underneath to corrode inwards.. one day 4 or 5 years down the road...
its going to break...AND THAT IS DANGEROUS !
MY ADVISE... SPEND YOUR MONEY ON THE BEST BLADE QUALITY, SOME GOOD OIL..AND PROPER USE AND MAINTANCE... YOU WILL BE FAR MORE SATISFIED....
OS