My brainy assistant pet acquired defender, which is good, but it only gives 3 block rating. Will this ever go up or am I doomed to keep this puny value forever?
Keep training your pet. As it goes beyond Ancient level and approaches Epic, you will see your pet's talents (like Defender) go up and up. By the time your pet reaches Epic level, you will see your Defender talent reach its maximum. Probably that will be somewhere between 25-30 critical block. The maximum possible level for Defender is 30.