Blue Art is the rarest and hardest skill to learn of the other four arts. It's rather difficult to really determine how a Remien is a Blue Remien but the most common clue is the death of their mother, the carrier. It also has the potential to cause massive heart failure to their father, but this was never truly proven but it has been recorded as a common trend of events. In more rare occasions, there will be a bright blue flash of light which is like a mini-bomb effect and anyone within a certain distance are bomb to dust. Yet it is known that some can survive the explosion without much of a scratch and it seems to have no connecting tread on as to why. Some say it is because they are meant to raise, or train them to adulthood. Other say is that they are the chosen relatives and kill others who were "directly" related to the infant. It is still remain in question and extremely difficult to determine correctly especially with the rarity of Blue Remien births.
Blue Art is possible to learn but extremely difficult especially to the other three arts. It requires you to physically be connected to the enemy or ally (or anyone in general), pick out their most treasure ability, draw it out mentally, and absorb it within themselves. The most difficult step to master is picking out their ability, because most of the time, the enemy or ally won't use it unless it is their last resort and the key to victory or survival. Those trying to learn the ability must be extremely patience and weed them out to their weakest point, wait for them to execute the action, and quickly absorb it. Now the most crucial step is to survive the action while at the same time preparing for it.
Once the ability is learnt, you can now use their skill against them and further more make it more powerful than they knew it was possible.