It's 1/3 of normal block, capped at 25%.
1) If you had 75% block, then running reduces it to 25%.
2) If you had 45% block then running reduces it to 15%.
3) If you had 120% block (capped to 75% on char screen) then running reduces it to 25%.
http://classic.battle.net/diablo2exp/basics/characters.shtmlJust for the record, the speed difference between running and walking is just +2 yards per second for runners. It is a flat bonus unnaffected by FR/Walk. If you're running at like 1/2 a screen a second (about 9 yards), then walking (at 7 yards per second) that distance would take 1.3 seconds. You probably won't notice that difference, but your block reduction and defence reduction will be noticed.
Maybe a Blazer sorc would still prefer to run, since they're exceedingly hard to kill via melee when they're in their patches, and since FR/Walk is a pretty decent damage (or area) buff for them.