To be fair, more Block Rating after you're uncrushable will still increase your chance to block while Holy Shield is down. This won't reduce your chance to be crushed though unless you somehow have 87.4% or more avoidence WITHOUT Holy Shield already so though it isnt entirely useless it doesn't do much for you.
Yea.. I'm all for simplicity, just thought I'd mention it because of the numbers. Incidentally, Block comes after all of the multiplicative factors, which means if you add up all of the sources contributing to Block Value (besides JUST the Shield as the OP asked) that will be a flat reduction on how much damage you take. Most mitigation sources that reduce damage by a fixed amount are applied before armor though. For real good information on the order of mitigation, the only real resource available is to check out every form of mitigation on wowwiki and see where it lies.