I've made a small fortune monetizing my poker blog by creating poker-related services and leveraging the trust built with my audience. I still collect significant revenues from this with no ongoing effort beyond answering emails. In fact, though my traffic was never above 30k uniques a month, I made more profit than many blogs that have uniques in the low millions, though it was also more work than simply slapping on ads.
Even off of ad revenue I was making low 4 figures monthly just by selling them directly. At least in the poker niche, when your blog gets big enough advertisers seek you out. I don't have enough experience in other niches to know if the same is true.
I have a feeling that anything gambling-related is easier to make money from. I had a similar experience with an advertiser (a British casino, iirc.) seeking me out when I made a silly little blackjack game.
Even off of ad revenue I was making low 4 figures monthly just by selling them directly. At least in the poker niche, when your blog gets big enough advertisers seek you out. I don't have enough experience in other niches to know if the same is true.