So when Zilean q's Malzahar, his shield breaks and the bomb still sticks and proceeds to blow up on him anyway. So what I was wondering is what exactly does Malzahar's passive block when it breaks?
It didn't block anything, malzahar's passive broke because he took 1 true damage from the bomb.
same thing for Trundle's pillar, these abilities does 1 true damage so the user can get assist.
"Void Shift phases out Malzahar, making him immune to crowd control and taking 90% reduced damage, lingering for 0.25 seconds after taking damage or blocking a crowd control effect."-taken from lol wikia.
I assume since Zilean's q is able to cc (through double qs), malz's passive takes this cc counter into account and breaks because of it.
Didn't they fixed it?
If it still does it's because Zilean Q does 1 true damage when it sticks to a unit, mostly done for tower aggro and assist purpose.
on the same note, 
can E a 
and as the slow will proc his shield, so if u wait for it to drop you can detonate it after the shield drops
As a zilean main, I think I know what this means. Whenever Zilean puts a bomb on someone under tower, he gains turret aggro, so the coding must be linked somehow. I dont see malz a lot any more, so I dont remember what happens when I put a bomb on him.
His shield doesn't 'block' anything.
It makes him effectively immune to damage and CC until it wears off; it wearing off means taking any hostile interaction. In this case, the bomb puts a debuff on him that deals true damage as that 'count down'.
It takes 1.5 seconds or something like that from the time he takes a hostile interaction until he's vulnerable.
