"You cannot pass. The dark fire will not avail you, flame of Udûn. Go back to the Shadow! You cannot pass."
Gandalf is addressing the Balrog as the Flame of Udun. Balrogs are Maiar of fire seduced by Melkor in the beginning of Arda, they dwelt in Utumno, the first fortress of Melkor. Udun is the Sindarin name of Utumno.
"Therefore Ilúvatar gave to their vision Being, and set it amid the Void, and the Secret Fire was sent to burn at the heart of the World; and it was called Eä."
The secret fire, or Flame Imperishable, represents the principle of existence and creation held by Eru. Gandalf's words identify him as a servant of the power of Ilúvatar.
"'You cannot pass,' he said. The orcs stood still, and a dead silence fell. 'I am a servant of the Secret Fire, wielder of the flame of Anor. You cannot pass."
Anor is the Sun, and so literally the 'flame of Anor' would be the light of the Sun. Since Gandalf claims to wield the light of the sun my opinion is that Gandalf is referring to the ring of fire. It's likely Celebrimbor used the light of the sun/trees in his making of the ring as his grandfather Feanor used the light of the trees in his making of the Silmarils.
Gandalf is indeed contrasting the dark fire of the balrog to the good fire of the secret flame and the light of the trees. These are specific things and not just metaphor.

