Alright then, I just want to add that I only pulled up that information to prove a point. I also actually only proved myself correct.
So let me address your last point.
Also, Madreds and Kog and J4 and a few other items/champs I can't think of ATM do %hp damage and it is widely known that the damage can be mitigated as well.
Kog's W scales off of AP. Meaning that it will do % damage (true damage) in addition a % of AP. You can counter the AP portion, NOT the % hp portion, unless you stack health.
As for J4, I assume you're referring to his passive. His passive caps at 400 max dmg. It is only for the first time you attack a target, and it can only occur once every 6 seconds.
Jarvan IV's first attack on a target deals 6 / 8 / 10 % of the target's current health as bonus physical damage. This caps at 400 damage. This effect cannot occur on the same target for 6 seconds.
Either way, this is no where near the amount of threat a Blackfire Torch can put out. Blackfire Torch's passive resets any time you take damage, including DoT abilities, for the entire duration of the ability. So that 5% damage could actually be something like 10-20% of someone's health. It is not inhibited by AoE abilities either. So in theory, if a Singed had this item he could deal maybe this amount of true damage to your whole team. The only other champions in which it may be an issue is Cassiopeia, Brand, and Teemo, or any AoE mage really. But the reason why it is not a problem for them? They're not tanks. Tanks are hard to kill
. He can pump out this kind of damage, while not being hindered in any way.
I'd also like to point that this item also adds Magic Penetration. Another item that can be deadly on a champion like Singed.