Wulf Ratbane
Adventurer
Somewhat ironically, one might allow blasphemy to be negated with silence, but not all groups are battle focused enough to realize such tactics.
Hmm... You could if it were tagged with the [sonic] descriptor (which it probably should be).
Special Abilities :: d20srd.org
It should also be caster-level dependent, not HD dependent.
Any spell that is not caster-level dependent, rather than HD dependent, is necessarily going to be skewed in favor of the monsters. PCs tend to advance caster level and HD at the same rate, with one rarely outstripping the other; whereas monsters typically advance HD much faster than they advance corresponding CR.
Saving throws for other effects, for example poison or breath weapons, are based on the typical 10 + 1/2 HD + ability modifier mechanic, which also tend to outstrip the PCs ability to meet the DC-- but these, at least, are based on one-half of HD, rather than the 1:1 deadly seriousness of blasphemy.