When I get 75 in Block will I be able to preform any staggering countermeasures against my opponents or do I need a shield? I'm using a warhammer.

Nope. The expert and master block 'perks' apply only to shields.

What you get blocking with a weapon is simply damage reduction. A master of block using a shield can block 75% of an incoming melee attack. That same master blocking with a weapon can block 50% of the damage. Oh, and of course the block animation (with or without shield) almost doubles your rate of spellcasting.

More here: http://www.uesp.net/wiki/Oblivion:Block

Well thats better then nothing. Thanks for the reply.