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Blasphemy is not a Hex Curse, no; it's a gem that interacts with Hex Curses.
Same with Bane.
The tags on both are accurate.
This is correct. The "curse" gem tag is intentionally only for skills that inflict curses, not for gems related to curses in some way. The "Hex" and "Mark" tags are used on gems that have primary interactions with those mechanics without actually being curses.