Hellblade is functionally a hybrid between two FF jobs, Dark Knight and Spellblade.
The Dark Knight aspect comes from using HP to deploy strong attacks and using the signature attack
Minus Strike* to deal increased damage as well as
Last Resort passive to further boost all attacks the closer their HP is to 0. They can boost it even further using Death Throes, which inflicts Doom on themselves. Dying gets extremely punished, setting Bravery Points to -3 (i.e. like getting Brink of Death on 1st death).
*PvP DRK has this signature within Bloodspiller as of patch 6.1+
The Spellblade aspect comes from using physical element-aspected attacks that deploy up to three elemental blades raining down on the enemy periodically as well as Ultima Blade, dropping the entire MP pool to deal monstrous amount of damage relative to MP spent (basically Despair/Flare on crack).
Facilitating all this it bolsters a high statline of HP, attack and defense stats as well as passive HP/MP regen. It also lets you learn the Break Damage Limit passive.
Hellblade's design in BD2 is a more demonic take on the classical FF Dark Knight look with the signature weapon being a black sword.