What is the Punishment for the blasphemers in Dante's inferno?

The souls of the sullen are punished for their lack of appreciation of the beauty of God's creations in life, therefore they are punished eternally to lie below the murky waters of the river Styx (the second river in Hell, though it is more like a swamp). As Virgil points out to Dante, the only way to recognize the sullen is by the bubbles of air that come to the surface from their chants.

"'Sullen were we in the air made sweet by the Sun;

in the glory of his shining our hearts poured

a bitter smoke. Sullen were we begun;

sullen we lie forever in this ditch.'"

~Unending shout by the Sullen (Dante's Inferno, Canto VIII, Lines 121-124)

If you analyze the punishment of the sullen, it is as symbolic as the others of their sin. While alive, they refused the light of the Sun (a symbol of Divine Illumination) so therefore in death, they are buried below the stinking waters of Styx, gargling Words of an endless chant that sort of mocks singing a hymn.