Fin do you know the base block rate of :
DA / PAL / SK / TK ?
http://l2p.l2wh.com/shieldblock.html (http://l2p.l2wh.com/shieldblock.html)
whole difference is only in DEX stat and guard stance skill
Base block rate of all shield is 20%. Off-hand parry dagger is 40%. Even with Hell Knife.
20% x ((DEX+80)/100) x Passive/buff/toggle/item bonus
Edit Corrected the formula. Dex mod is (dex+80)/100, not dex+80/100.
I haven't checked if the rate would be doubled when attacked by bow or critical attacks, including blows like in official.
The efficiency of bless shield had been decreased at T1 (it's not updated on L2P) but for bow and critical, tanks can block bow and critical attacks 100% by shield only with shield mastery lv.4 and bless shield in official. (I mean not about damage. It's about shield block rate.)