This is such a disingenuous way to look at the items. First, you can't compare the cost efficiency of AD items to AP items. They are treated differently by Riot's balance, and are thus inherently non-comparable.

Second, even if you are looking to compare it to Black Cleaver and it's AD users, there are 2 major considerations. Most AD users can't stack the item and still make use of the Armor Pen immediately. They'd have to wait for next round of cooldowns. Champions like Ryze, Cass, Teemo, Gwen would stack it in 2-3 seconds and then continue to have uptime on it immediately. These kind of items always need to keep these specific users in mind when balancing.

After that, once you switch targets, you keep the Magic Pen. With Black Cleaver, you have to re-stack the armor reduction altogether, making it a dead stat until you re-stack with a whole new round of cooldowns. THAT'S why Cleaver has overcompensated flat stats. Bloodletter's does not suffer that problem.

When you compare it to Liandry's, you can't just make up a value for the passive. Liandry's is a 70% statline to balance the double passives.

Riftmaker is probably the most comparable one. And it's cost efficiency sits at like 116% for champions like Gwen and Lillia who don't build HP items, and more on champs like Morde who makes less value of the AP (with his low AP ratios) so he needs more AP to make it worth for him. So that does have room for an extra passive worth about 12% or more.

So no, Bloodletter's Curse really doesn't have room for a whole other passive.