Blade exists largely to target health stacking opponents. So, why not make it designed to target opponents who stack health?
Buff the Blade of the Ruined King's base stats just barely so that it's not a total money sink if you are fighting against somebody who doesn't stack health at all, and then rework (and buff) its current "Total HP" damages to "Bonus HP".
This creates a strong niche for the Blade of the Ruined King now. It's now subpar as a catchall item you can buy for any situation, but (assuming you reworked the bonus HP damage correctly) it's now very well designed as an item to kill health stacking champions.
(It also fixes the problem of how some Assassins use Blade of the Ruined King. Seriously, most of the time, Assassins are looking for the squishiest members of the opposing team. Why the f- are they buying an anti health stacking item against intentionally squishy targets?!"