Passive income is any income that you make without having to do much work to bring in that money.
Rental income, investments, website and blogging income are examples of passive income.
Although they do require some work to get it going and keep it going they don't require much work.
Blogging will certainly require a lot of work in the beginning before you start making passive income from the blog but overtime you won't have to work as hard to keep the money coming in.