The wording of Blast Burn reference discarding Energy Cards (2 for Heads 4 for Tails). If you had Charizard with 2 Fire Energy and a Double Rainbow Energy, you would have enough energy for the attack. But, if you flipped tails, you could discard only 3 energy cards hence the attack would do nothing. Note when it says nothing that also would mean you wouldn't discard any energy. Nothing means nothing ...
Hopefully that clears things up