At first it seems you are asking to create an extension for Chrome that will block sites. To this I would say, "Sure, go ahead, but why Avast!-brand it?
But it seems like maybe you want it to be Avast! branded because.....you want this to be something that would allow ordinary end-users to choose what sites to place on the Avast! Network Shield's site blacklist? If this is what you are shooting for, I have to tell you (though its no "official word", I dont work for Avast!), I don't see that happening. No single end-user should have the responsibility of saying what sites are bad for all.