Block value in classic WoW is the flat amount a shield takes off a blocked hit; it is the shield block value plus one point for every 20 Strength.
damage blocked = shield block value + strength / 20
A block takes a flat amount off the hit, not a share of it: the shield's block value plus one point per 20 Strength.
| Shield with 40 block value, 300 Strength | 40 + 15 = 55 damage blocked per block |
Last checked: unverified against Forever. These are the long-established values of the classic ruleset (the 1.x game of 2005-2006); World of Warcraft Forever is in beta and may differ. When you test this one in Forever, post your numbers in Theorycraft.