If you're a paid subscriber then I think it's morally OK to use an ad blocker on them. You're paying for what you're consuming, and in all likelihood you're only earning them a few cents to maybe a dollar from ads anyways. What you spend on your subscription drives way more value.
Something I haven't though of before - with paper newspapers, there is no way to opt out of ads, and those are also subscription based. Why is the internet different? There is no opt-out for newspapers. Presumably they are getting less revenue than the customer agreed upon if the ads aren't displayed or aren't clicked on. Is using an ad blocker effectively stealing?
Note: I use an ad blocker primarily as a means against malware.
If internet ads were as unobtrusive as newspaper print ads (I'm not talking about the shitty free tabloids), a lot fewer people would be using ad-blockers.