If they went by their 3.5 ruling, bless/corrupt weapon would just do +1 EB vs their opposed alignment and auto-confirm crit rolls as previously stated. You could only reasonably represent the former part in NWN, which would make both abilities almost completely redundant.
I do agree that bless/corrupt weapon should be parallel, but unconditional 2d6 negative damage is pretty strong as a 1st-level buff, even if you have to jump some hoops to get there by becoming a Blackguard.
I would kind of like to see them become their respective counterparts i.e. Bless Weapon is 2d6 divine vs. evil, and Corrupt Weapon is 2d6 divine vs. good. As mentioned negative damage is a lot easier to resist than divine. But, this comes with the problem with indirectly buffing Paladin, which is a class that seriously doesn't need it imho.
I'm all for Blackguard buffs in some form or another though, ironically I don't think everything Blackguards do should just mirror what paladins do but bless/corrupt weapon seems to be an intentional contrast.
edit: also just to clarify, as someone who hoarded Blackguard/Paladin scrolls like candy, they seem to be exceptionally more rare (as they should be really, they're really good spells).