I think you can only block light melee attacks. So you'll look pretty silly spamming blocks if your opponent whacks you with their tail, a nasty wristblade uppercut or shoots you.
The new gameplay videos look amazing, I'm so stoked for this game.
Agreed that the Alien first-person melee anims need work, but I don't have any problem with the concept of them blocking. The critter in Alien 3 did it when Ripley went at it with a pipe, if I remember rightly.