On my opponent's last turn, they flipped heads for Diglett's "Dig" attack. If I use Blissey V's "Blissful Blast" attack on my turn, would I be able to attach Energy cards from my discard pile?
No. Since Blissful Blast did no damage due to Dig, you cannot get any Energy cards from the discard pile with Blissful Blast.
Blissful BlastDig