The ability to use both magic and ki equally as well as simultaneously. That was considered impossible by anyone, given what it would require, as that would mean someone could use both their internal mana and take in external mana at the exact same time. The limits of a mage are that, despite having the ability to channel massive amounts of mana through their bodies, they can not retain that mana within themselves, and they can not convert their own internal mana into ki for usage that way. The ability to use external mana(that being the mana that exists in the world itself) gives them the ability to use larger, powerful, and long range attacks as well as support. Something that a weapon user can not do.
A weapon user can only convert their own internal mana into ki, thus mostly use it to enhance their own bodies; only those with the title of master can do anything even close to just enhancing themselves, their weapon, or use the few distance attacks they do have. While we can not fire off ki like a blast or project it outward like a mage is capable of doing. What being a master (of any weapon, regardless of what it is) can do is use their weapon to produce various skills to do various things. Like swing something so fast it can create a vacuum, create a short distance slash of wind, and other things. They are replicating various things that are possible but would take someone's masterful amount of skills, training, and timing to pull off. Unlike magic, which can be taught to anyone if they have the desire to learn as a mage. Skills using ki are something one has to just have both the talent for and the will to train to obtain them. One can have the talent to use skills as a weapon user, but without the drive to train, they will never be able to do them. It also works the other way as well, as just because someone has the drive, if they do not have the talent, then they will never be able to use the skill they are attempting to learn. This does not mean they can not learn any skills at all, just that they can not learn THAT one particular skill, regardless of anything they do.
That is why many think that the Path of the Mana Blade is merely a fairy tale that was made up by someone long ago. For whatever reason is unknown, but it is unlikely something like that could have ever truly existed. However, those who explore and search for signs of the opposite being true have a different view of things compared to others. As there are many who think the fairy tale for Path of Mana Blade is fictional, it causes them to question something that caught their attention. If something is fictional, then most of the time, there is something at the very core of the tale itself that is true, no matter how small it is. After all, the Path of the Mana Blade had to be inspired by something, as it did not just come into existence out of thin air, as it must exist in some form. As the creator of the fairy tale, it was probably based on reality, and that is what was built up and developed into the fairy tale that they know now.
Mana existing in its pure form is a clue, as there are mana users that are neither weapon users nor mages after all. Those cases are very rare, as there are so many of them that, rather than have an individual name for each and every one of them, they are just collectively called mana users if they are neither a weapon user nor a mage. Though they normally lean closer to one side of the spectrum or the other, making it so that they can be mistaken for one or the other. Those who can use mana to heal others or themselves are more likely to be considered just mana users.
Like someone who can produce claws made of mana from their hands and feet, but no matter their appearance, they can not actually harm another with them at all. They literally stab a person full in the chest with their mana claws, but that would not cause the person any damage at all. The claws are not meant to do any damage at all, but instead, this person would be a very unique and direct healer who heals through the usage of their mana claws. How or why they have this ability is something that causes the truth of the Path of Mana Blade to be questioned.
If something were to be possible, then it would and/or could suggest that the truth of the Path of the Mana Blade lies in finding a person who is as close to the center of being a weapon user and mage as possible. Trying to find one person who can use both magic and ki equally as well as simultaneously is akin to looking for a naturally born unicorn. Something that might be possible, but you would have to be there at the exact moment they are literally born into this world to prove that they are indeed naturally born as a unicorn.
This is where one mage came up with an idea that they figure might lead them at least in the right direction towards the truth of the Path of Mana Blade. As there was a phenomenon in this world where one takes in and/or stores too much mana within themselves. It could cause the tanning of their skin as well as cause their hair to turn white. However, just like with all other phoenmonas that cause the skin to all of a sudden change to a different color, the amount of mana, just like in those cases, must be an extreme amount. Thus leading this one mage to look for those who have experienced this phenomenon to see what they are capable of. If one were truly capable of using both magic and ki equally as well as simultaneously, then this phenomenon would not happen to them. Though looking for those who have gone through it could lead to something just interesting.