The problem with duskblade is there is no counter to it, not really. It adds bonus damage while negating armor, you know, that thing meant to counter physical damage? Yes, its not equal to the full armor value of a defensive item, but still actively works against the mitigation meant to counter it.
As for the -unseen- passive, you can't stop someone from going into FoW. At least with hexdrinker I can pop the shield then wait for it to clear, but it is impossible to stop the enemy from leaving vision. I feel like duskblade wouldn't be nearly as much of an issue if its proc condition was the same as serrated dirk, the killing a unit requirement. This would cement it as more of a laning/combo tool and less of an all-in all the time tool that has no real set up condition. This idea also has synergy with the ward clearing aspect of the item, allowing you to proc the passive while also clearing enemy wards in your lane for a gank.
As duskblade stands now, it is never a bad item to buy, ever, on any attack damage character.
is cheaper than 
(most of damage champions build this first item) ?
synergizes way better.