Dispel Magic is good, but in this specific instance, that is reducing damage and the to hit percentage, Dullblade is better.
If facing an opponent with a Bladesharp 4 on his sword for example, a Dispel 2 will have no affect (and would be a waste of magic points and of an action), while a Dullblade 2 will reduce the damage.
BTW, something I just spotted under Bladesharp description:
" If Dullblade is also cast on the weapon, the improved chance to hit from Bladesharp is not affected, but the weaponโs additional damage is reduced by the second spell."
Does that mean that if you cast a Bladesharp 1 and then someone casts a Dullblade 4 the total affect on the weapon is +5% to hit and -3 damage?