A Taekwondo soodo marki (or shuto or knife hand) performed in back stance has the pull back hand palm-up in front of the solar plexus and the extended hand palm-forward.
In the Taekwondo form Dan-gun, the blade of the hand is the weapon that makes contact with the target. However, I have not seen students doing any hand conditioning to the point where that blade would be hard enough to cause damage to an opponent.
20 years ago, my first martial arts instructor started us on conditioning equipment and poultices which thickened our knife hand and shins. I don't train the edge of my hand that much anymore but I can probably see evidence of this training if I look close enough.
One of the equipment my instructor introduced to us were large white candles. We would roll our knife hand blades into the candles that were in turn placed flat on the surface of a table. Then we would roll the entire blade back and forth pressing downward. After a few weeks of this we would gently 'smash' the candle to bits whilst it lay flat on the table.
My students nowadays have the option to pursue this, so I do highlight it as an optional part of their training. However, considering that most don't actively condition their hands, I have promoted a different soodo to the one that is commonly practiced in Taekwondo and Karate halls. This is a variation where we use the corner of the heel palm rather than the blade of the hand.
All you've got to do is to flex the hand towards the thumb side of the forearm when the shuto is extended in front of your face. Then you make a very shallow hollow in your palm as you tense the muscles in the heel of your palm. The best thing about this type of fist is that you can create a fairly formidable self defence tool easily and immediately. Try smacking the ground with the front face of your knuckles held in a fist. Then with your shuto. Then with the heel palm. Then with the corner of the heel palm. It should be fairly easy to dinstinguish which out of that lineup hurts the least.