This is a really tricky question, but I believe I have your answers. Apologies in advance for the long-winded explanation:
Damage shields do not have a mitigation table. They don't have an "armor" or "spell" resist; rather, they are simply a flat value. That flat value is derived from a few variables, namely the primary stat that it scales from (i.e. max magicka for harness magicka, or health for bone shield). In a hypothetical scenario, imagine having a 10k shield active, and doing nothing else to attempt to mitigate the incoming damage. An enemy swings at for you for 5k, and no matter how much armor/spell resist you have, it's still a 5k hit to the shield, which is now effectively reduced to half.
Let's take that same example, but instead of using a 5k attack, we'll use a 15k attack. We know that this amount of damage exceeds to value of your shield by 5k, which will then be subject to your various mitigation values. Your armor/spell resist will reduce this 5k, as will blocking and any racial/class passives. Here is the key: blocking, armor/spell resist, and racial mitigation passives do not reduce the incoming attack before it hits your shield. It comes into play against the remaining value of the attack after the shield is used up.
One exception to this rule is the application of debuffs. In most cases, the debuff you will be concerned about here is Minor Maim (talons, low slash, shades). If your enemy is Maimed, the damage they deal is reduced. So in the first example above, that 5k attack becomes (.85 * 5k) = 4,250. It is for this reason that you will see some tanks apply Minor Maim to a target prior to group-wide mechanics, like the Mantikora's raid-wide, or the "starfall" ability used by the Warrior, etc. It helps to offset the damage being dealt to the entire group.
TLDR: For your question specifically, a "hit" against your shield is, for all intents and purposes, a "hit" against you. If you have a shield active, and an enemy deals damage to the shield, it will proc Constitution (assuming it's not on cooldown, I believe it's 4s off the top of my head). The same applies to Wrath. If your shield takes a hit, you "get credit" for it, effectively boosting your damage and restoring a bit of resources.
However, an example of something that is counter-intuitive to Bone Shield is Major Evasion. Every attack you dodge is an attack that did not hit. Therefore, dodging an attack will not proc Wrath, nor will it proc Constitution. Don't let this deter you from using Major Evasion, though. It's one of those stats that is very important for tanking, but something we have to be consciously aware of. In some cases, it may be unwise to use it, but that decision is up to you and is mostly contingent upon your gear. Just keep in mind that even with Major Evasion, you have ~80% chance to be hit by an attack, which you will want/need to offset in some way, either through shields, mitigation, or a combination of the two.