How effective do you think a blog would be if you just wrote content and never used any marketing for it? You just let google index it and people will find it through google.
If you have unique content which is helpful to people, it will work. Just will take more time thats all.
If you do absolutely no marketing, your site will never appear in Google for anyone to find. To get into the index of any of the major search engines, you need at least one solid link from a well-ranked website. To get that one solid link you need to do at least a tiny amount of marketing - even if its posting the link yourself on another website or asking a friend for a link. Beyond that simple indexing, to get your site to appear high enough in the search results for anything but the most obscure search terms, you'll very probably need more than that single link, so you'll need to either hope that the direct traffic from that single link turns into some additional links, or do even more marketing. But with truly zero marketing effort, your site will be effectively invisible.
If you have unique content and optimized with the right keywords your blog can get good rankings and traffic. But you still need to get quality links so that google will find your site to indexed.