Not sure if anything has been clarified on this and my math isnt exactly solid... but from what i've seen, Blood Shield Mastery has HEAVY Dminishing Returns once it's at 100%.
I was at 101% and stacked a ton of extra Mastery... this caused me to only gain an additional 2% mastery to put me up to 103% when before 100% the same amount of mastery would have netted me atleast 6%.
With 0 Mastery rating (read 8 Mastery) I put on my chest with 182 rating, which gave me 1.29 Mastery. (from 8 to 9,29)
After that I put on all my gear except for the chest and was at 1528 rating which translates into 16.52 Mastery.
Putting the chest on got me up to 17,82 Mastery (probably a rounding issue) and the absorb of the shield went up from 130% to 111%
Edit: Oh and Ratingbuster seems to calculate it wrong.