Can a blocked septum cause bad breath?

No: Deviation of the septum will only cause nasal obstruction. Bad breath could be from sinusitis which could in part related to poor nasal breathing from a septal deviation. However, likely from another cause (tonsil stones, gum disease, dental caries, ..).

Yes...: Specially if the blockage is associated with bacterial growth, or sinus backflow drainage. See an ENT specialist.