Does Hydreigon's "Dark Aura" Ability override the original type of attached energy, or is it in addition to the energy's original type?
Dark Aura replaces the attached energy's original type or types with Darkness type. In other words, a Fire Energy card no longer provides fire energy when Dark Aura affects it.
Source: Noble Victories FAQ (2011-11-17), PUI Rules Team (2011-11-17)

Relevant cards

Note: All applicable cards may not be listed below.
WOTCE1_29
WOTCE1_66