I made a new sentinel and I noticed I had a Slab in my stash. It has no level requirement so I put it on my new character. I have not blocked a single attack since equipping it. It’s supposed to be a 40% reduction in block chance but it appears to be making block chance 0%. For what it’s worth, my character sheet says 18% but as I said, I haven’t blocked a single attack since equipping it and I have gained 3 levels in that time.
edit: Went from level 3 to level 8 without a single block so I re-equipped my starter shield and immediately started seeing blocks again. Item is definitely not working.