The Blastline was formed in 3040 AD due to the Grathnode Inferia catastrophe. The shattering of the Grathnode Discs of the First Tower of Ar tonelico that caused the catastrophe also released massive amounts of plasma into the atmosphere, which created this sea of plasma that encircles the entirety of the Planet, pretty much sealing off all access to the outer reaches of the atmosphere to all airships built at the time.
Many brave engineers and pilots have tried inventing and piloting airships that could cross the Blastline, some even making this the point of their lives, but only a few have been successful. Examples among them being Luke and his ex-girlfriend Krusche Elendia, the latter being the engineer and pilot that helped Lyner Barsett's party to cross the Blastline in Ar tonelico: Melody of Elemia to reach the top of the Tower.
The Blastline hasn't gone off following the conclusion of the Planet Regeneration Project, as its main cause is a massive leak of energy because of the damage the First Tower suffered during the Grathnode Inferia. Still, the Wills of Planet Ar Ciel are doing their best to neutralize it with the same Song they are using to re-create life in the surface of the Planet, so while its effects will be greatly diminished, it would be necessary to repair the Tower for the Blastline to be completely erased.
The Blastline itself is located at 200 Stons (around 620 meters) above the administrative center of Sol Ciel, Platina, and due to being a sea of plasma, any living being that enters it without protection will have the water from its body gradually evaporated until the point in which they will die from dehydration. This is why no people can withstand being inside the Blastline for long periods of time, although it seems that the inhabitants of the Dragon's Nest have managed to adapt and can survive inside of it without much problem.
Likewise, given the Flipper-based mechanisms the current airships throughout Ar Ciel use, the airships can't go through the Blastline either. This happens because the massive amount of energy distorts an essential mechanism of the airships called the Liku Plate, which is the one that endows the Flipper with kinetic energy. Inside the Blastline, the Liku Plate's rotation is distorted, which makes it unable to send any energy to the Flipper and this sends the airship crashing down upon making contact with the plasma. This is the reason why the airships capable of crossing the Blastline have a secondary power source called the Nabite Cap, which serves as a Symphonic Power battery that activates in situations where the Liku Plate can't function correctly. However, the airships can't be in the Blastline for too long even if they are equipped with the Nabite Cap, so it's advised to fly through it as fast as possible.
Aside of this, the Blastline also had a secondary use for the Planet: with the creation of the Antibodies, She could send them to draw energy from the Blastline and send it to the Planetary Core as a way of sustaining Herself after Suzunomia was banished from the Wills' Consensus due to being declared as useless. However, even this wouldn't be enough to keep the Planet alive.
Finally, the Blastline has variable levels of intensity depending on the geographic location, being at its weakest over Sol Cluster, where its energy has mostly dissipated, and being at its strongest over Sol Ciel due to being the ground zero of the Grathnode Inferia. The area of the atmosphere it pollutes equals roughly to the broadcast range of the First Tower of Ar tonelico, explaining why there is such a large discrepancy between its intensity levels from a region to other.