The myth of blood grooves
Why do blades have blood grooves? Answer: - to allow air or blood to escape from a STABBING attack because it will enable you to with draw the blade from the body.
However the samurai sword was made to cut or slash not stab.
So what are these grooves for ?
Answer: - The grooves (hi) exist for 2 reasons.
1) To reduce the weight on the heaver blades making them easy to deploy. By using this method the balance was undamaged.
2) Grooves were also used to cut defects out of the blade occurring from the forging process. This could happen to the best sword smith on occasion.