Ok.... After watching Chad's video and hearing about this I was curious for myself if it was really hype or if there was something to it. I did a little more research on the subject of cryogenic tempering and found out a few things.
First let's start with the normal hardening process.
The simple version is 3 steps. Bring the steel to a specified temperature, hold it there for a specified amount of time, then bring it back to room temperature.
The differences of the steel alloys and the particulars of those 3 steps is where it gets complicated and has a ton of variables.
For something basic like I do, let's say 5160 medium/high carbon steel. You take the steel to 1575 degrees fahrenheit and hold that temperature for 3-5 minutes. This allows austenite to form. Then it gets quenched or rapid cooled allowing the austenite to change to martensite. If it is allowed to cool too slowly, it turns to ferrite instead and you don't get a hardened piece of steel.
After the quench, the steel is very hard, but brittle. The rapid cooling of the quench has frozen the atoms in place and often under a bit of stress. Tempering is the method used to help the steel relax a little and let the grain structure realign making it stronger.
Generally this is done by heating the steel again, but at a much lower temperature than it's austenizing temperature.
Some alloys like the martensitic stainless that I mentioned in Chad's thread use a cryogenic process where the steel is heated, then put in a deep freeze, then heated again.
Now on to what I found out about the cryo-tempering in question. This is the part that I wasn't sure about so I decided to look into it further so I wasn't spreading false information.
If the original heat treat process is done properly and there were no issues, most tool steels/high carbon steels have about 99% martensite. However if things don't go as planned there could only be, let's say an 80% transformation leaving little pockets of austenite. These little pockets create a weakness in the material.
The Cryogenic tempering process brings the material to +300F (or maybe more depending on the specific alloy), rapid cools it to -300F, back up to +300f, then to room temperature.
The cooling to -300 allows fresh martensite to form, but being brittle similar to fresh from a quench, it is brought back to +300F to allow for the realigning/strengthening phase similar to the initial tempering.
So the process doesn't necessarily make a "harder" steel, it makes a denser more stable material by converting the remaining austenite to martensite.
So for things like milling cutters or other tool steels, there can be a significant benefit to the longevity of the tool. Probably even for knives, and I suppose to some extent a razor blade.
I personally do still feel that for a razor blade, it is still gimmicky. The razor blades are so thin that the benefits of it really wouldn't be that noticeable. I'm also not sure how they would treat the blades in the package like the link Dave shared without any deformation to the plastic. Many polymers start to melt at 300F.
However, I did learn that the process is a real thing after the main heat treating process. So I stand corrected. But, I have furthered my own education and made my brain hurt.
