During a recent game meetup I used Teleportal to give all my creatures unblockable. Another player responded by targeting one of my creatures with Shriek of Dread to give it "can't be blocked except by artifact creatures and/or black creatures."
We decided (reluctantly in my case) that this might override the unblockable. Our reasoning was that because the revised wording of unblockable is so similar (can't be blocked) the exception would apply to both instances of it.
Are we right?