Block

From Krakiipedia, the unofficial free GemStone IV encyclopedia.

To block (in the context of the Evade/Block/Parry system) is to outrightly prevent a physical attack using one's shield, stopping the attack before the attack equation has even come into play. The check to determine whether an attack is blocked occurs after the check to determine whether the attack is evaded, but before the check to determine whether it is parried.

Chance to Block

The odds for an outright block are determined by comparing the defender's Shield Use skill to the attacker's level. Against a like-level foe, 1x training will yield about a 5% chance to block in stance offensive; 3x training, about 15%. Other factors that affect the chance to block include:

The passive combat maneuver Block Mastery will increase this chance by 5% per rank.
shield size
certain stats (probably strength and dexterity)
stance
positioning: Kneeling reduces one's chance to block by 25%; being prone reduces it by 50%

While a shield does increase one's bolt defensive strength more than one's melee defensive strength, it is not normally possible to outrightly block a bolt spell.