When it comes to Zororak it's important to note that the attack cost of the opposing pokemon is irrelevant, but the effects associated with the attack are always taken into account.
In the attacking process, the attack cost of the attacking pokemon takes priority over effects of the attack. Without meeting the cost of the attack on the card the attack cannot be used in the first place.
Reshiram Needs RRC to use Blue Flare, so you have to provide 2 fire and 1 of any other energy. Then the effect of Blue Flare activates and it says to discard 2 fire. Reshiram has 2 on it, so the energies are discarded and the effect resolves.
With Zoroark, Foul Play reads "Choose 1 of the defending pokemon's attacks and use it as this attack." It lacks the text on cards like Mew that read "(You still need the necessary Energy to use each attack.)"
So Zoroark's attack process says it needs CC to use Foul Play to use Blue Flare, so you can provide any 2 energy. Then Blue Flare says to discard 2 fire energy, and if any of the energies are fire(basic/rainbow/blend, whatever), they are discarded. If not, there isn't text saying the attack won't work if the energies are not discarded. So the energy discarding process only happens if it is applicable, then the effect resolves.
I really hope I explained that well.