That's another issue; blacklist limitation. It used to be that Cox limited the blacklist to 50 entries. I know they bumped it up, but I'm not sure what the limit is now. I've bumped into that limit in the past, but I've been able to add more in recently, so it must have gotten expanded when they moved to the out sourced provider.
The blacklist is buried in the settings and is a *** to find. Its doesn't matter anyway, because you can no longer edit it. The only changes that you can make is to delete an entry. Which I suppose if you've hit the limitation, perhaps you might want to remove some old entries. However, I wouldn't bother. The email addresses are all spoofed, and what gets added will never block any inbound emails.
I used to dissect the email header, find the real email address and enter either it or the domain that it was sent from, but is getting impossible to figure out anymore and since you can't manually add in an entry, it doesn't matter.
Daryl