In WOTLK, all Paladin seals have been changed to be 30 minute, undispellable buffs.
However, Seal of Corruption, the new Horde-only seal added in WOTLK, has been implemented as a dispellable magic buff.
This is inconsistent with the rest of the seals, which all show as non-magic buffs, as well as its Alliance-only counterpart, Seal of Vengeance, which is also correctly a non-magic, undispellable buff.