Currently if you block while you have a damage shield you will still incur the cost of blocking an incoming attack, even if the damage is entirely absorbed by your shield. I can only assume this is "working as intended" but I would really like to see this changed. Yes you could always drop your block while your shield is active, but this makes small class damage shields like Igneous Shield (DKs) and Blazing Shield (Templars) cumbersome because they can be popped very quickly so constantly be blocking\unblocking if you want to save stamina by spam casting those abilities. It also doesn't make any sense from a logical point of view that an attack that isn't block by your character, but rather absorbed by a magical shield somehow costs your character stamina...
Another thing that I consider just a bug is that you're vulnerable to Molag Kena's conal pushback if the attack is absorbed by a damage shield even while blocking. Its getting very annoying being pushed back by her as a tank just because I cast Igneous Shield a second or two before she does that attack.