SEO's probably the most powerful traffic solution for blogs.
Paid ads won't work because even if you do affiliate marketing on a content-rich blog, most blogs aren't just "SELL. SELL. SELL." If it is, it's not a blog - it's just a affiliate landing page website.
Blogs require more organic traffic (i.e. trust) for people to buy goods. As a result, if you slam your blog on google ads, you'll pay tons of CPC but not really have a loyal audience. Whereas in SEO your traffic is free in money but expensive in time -- but people who organically discover your website and stick around are more likely to trust you enough to buy from your links than someone who ran into your blog via ads.
Again, if you have a website that's purely devoted to affiliate marketing (i.e. just a landing page that tries to sell one product, with a long-winded sales letter), that's a different story and something like Google ads / Bing ads could work.