no, there is no such thing as +maximum % chance to block sorrily

blockrate is the number of frames per 25 frames it takes you to performs to the blocking animation once.
differentclasses have different speeds, paladins being the top
then barbs
then zon/assassin
then druid/necro
then sorcs
the barb and pally can reach a maximum of 3 frames to block
while zon/assassin and below can only reach 4 fbr
more inc block rate means you have a faster reaction time and reduces the amount of time it takes you to move after blocking
it prevens "block lock"

hope this helps =-)