First, the important part of dispel magic:
Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends.
Targeting a creature
If you target a creature, your only option is to (attempt to) dispel every spell on that creature. In this case, “any spell (…) on the target” is intended to be read as “each of the zero or more spells (…) on the target”.
Targeting a magical effect
The rules on targeting spells say:
To target something, you must have a clear path to it, so it can’t be behind total cover.
This doesn’t just apply to creatures, so you must have a clear path to the magical effect in order to target it. The standard cover rules (including weird edge cases like windows and wall of force) have been covered in many other questions. The interesting question for dispel magic is: to which magical effects can you have a clear path?
You certainly have a clear path to shield of faith, since a “shimmering field appears and surrounds a creature”. On the flip side, nothing in the description of death ward indicates that it is present in any way; after all, spells only do what they say they do. I would therefore conclude that death ward cannot be directly targeted, because it isn’t present in a way that allows for a clear path. You could still target the creature, just not the effect itself.
Unlike many other spells, dispel magic allows you to choose any target, even one you can’t see. That said, you still have to choose a target.
Spells that produce a visual effect (blur, fire shield) are clearly choose-able. Any creature affected by zone of truth is “aware of the spell”, and thus could choose to dispel it, despite the lack of visual effect. A creature that correctly identifies a spell (via an ability check as it is cast or identify) could choose to dispel an otherwise-imperceptible spell, like aura of vitality, so long as the creature has a clear path to the effect.
The rules don’t say that you need to know exactly what spell you are dispelling. Therefore, dispelling “the 7th-level hallow spell I just identified” is just as acceptable as “whatever these tentacles are”. If you try to dispel something that isn’t a spell, it just doesn’t work. Similarly, if you choose “the invisibility on the creature in that square” and there is no creature in that square, it just doesn’t work.
In order to dispel an individual effect, you must both…
- Have a clear path to the effect.
- Be aware of the effect (enough to precisely target it).
In your specific example, blur qualifies for both points. You can certainly be aware of slow, but a clear path is trickier:
You alter time around up to six creatures of your choice in a 40-foot cube within range.
Unlike, say cloudkill, slow doesn’t actually create an effect that fills the 40-foot cube. Rather, targets are chosen from within that area. That said, the fact that you “alter time around up to six creatures” leads me to believe that there is an invisible area of slowed time surrounding each target. Therefore, you can have a clear path to the effect. (The wording is really important here. If slow said that you alter time for creatures, I would say that you don’t have a clear path.)
As a result, you can dispel slow or blur selectively (by targeting an effect), or you can dispel them both (by targeting the creature).