The duskblade is a combination of arcane caster and melee fighter. They typically serve as support characters not being able to specialize in combat or casting as certain other classes can.
In Eberron, duskblades are most likely come from Aundair, where they may have served to support war mages cadres, such as the Ninth Wands, in the Last War. The may also be trained to serve at Arcanix, to guard the wizards there in a physical capacity, but also requiring, due to the nature of the place, to have an understanding of magical forces.
Others may be Cyran refugees, who suffered seeing their nation die on the Day of Mourning, but survived due to the combination of their arcane knowledge and physical abilities, which allowed them to avoid or combat the likes of the living spells and wandering monstrosities both.
References
- Player's Handbook II. David Noonan (2006). Wizards of the Coast.