If Flareon's "Flare Effect" ability or Vaporeon's "Aqua Effect" ability or Jolteon's "Electric Effect" ability adds Fire or Water or Electric type to a Darkness Type Pokemon, does Dangerous Energy get discarded since it says, "If this card is attached to anything other than a {D} Pokemon, discard this card"?

No, it is not discarded. Even with the addition of another type to the Darkness Pokemon it is still also a Darkness Pokemon.

Relevant cards