I'm not sure whether a blog should be considered passive income or "earned" income as you put it. Stop posting and promoting your website, the visitors will dwindle and advertising will dry up. That's probably not the same for all types of websites... but I think continuous promotion is the key. And that takes time, and while you're not getting paid for that time, you wouldn't get paid without putting in that time.
On the other hand... income from investments is truly passive because once you make the deposit, you can just sit back and enjoy the interest without any maintenance.
The same way, income from rental properties only becomes passive when someone else is managing the properties for you. If you have to be a landlord, find tenants, and fix sinks... your trading your time for money.
As long as you plainly disclose which posts are being paid for and which are not... then those who don't like reading paid-for blog entries can simply stop reading your blog if they desire.
