Since the default block extender has a empty blacklist (or the functionality of one, anyway), all other block extenders should also have this. At least one reason for this is to allow block extenders to work with no configuration.

If you think differently, please tell me why.

It was like this because I also though upgrades should not immediately change behavior, but a lot of people expected 'whitelist' to be the default option (as most MC filter systems work like that), so at request I changed it to what it is now. I'll not change it again :P

Thermal Expansion's itemducts have default blacklist, what sorting systems were they referring to, do you know?