Block rate is luxury, and generally specific to classes and builds that can hardly get any defense. You need a significant amount of dexterity investment to reach maximum block, depending on your base block rate. One of the best ways to get a good amount of dexterity, besides charms, is using abandoned crafts, blessed as well. As well as sockets filled with amyphysts or Io runes.
Quote from docs on block:
Block
In Median XL, your chance to block depends on character class:
Amazon/Assassin/Sorceress: 3%
Druid: 2%
Paladin: 1%
Barbarian/Necromancer: 0%
Shields have 0% chance to block, except class-specific shields – which provide +1% extra chance to block.
The block rating is boosted by dexterity as follows:
Chance to Block =
Block% * (Dexterity - 15)
2 * Character_Level
Capped at 75% chance to block, rounded down.
Block% is the base chance to block from: the character class + class-only shield + other bonuses (items/skills/etc.)
Max block is hard to reach; you will need to invest heavily into dexterity. This is a balance measure in Median XL to ensure that shields do not outclass two-handed weapons as in classic Diablo II Lord of Destruction.
Amount of dexterity needed to achieve max block (75%) on a level 120 character:
Base block Dex needed
1% 18015
2% 9015
3% 6015
4% 4515
5% 3615
6% 3015
7% 2587
8% 2265
9% 2015