How to dodge knife attacks?
There is no direct counter to the knife goons in Arkham Asylum.
You may JUMP over them to continue your combo to another goon, or you can CAPE STUN them to open them up to direct attack.
A good strategy when fighting a group of enemies that contain 'special goons' ( knife and stun wielding ones ) is to build up your combo until you have a yellow move and do an instant takedown or body throw on the blade goons.