Here's stats for a black niche blog. It's not a lot of cash compared to other methods or business models, but it's nothing to sneeze at either. Blogs can definitely be a way to DIVERSIFY your traffic/promotions method portfolio. However, you still have to work at getting traffic to your blog.
Take Note: There's no magic bullet re income in online promotions. A company or individual might get you the tools you need, but ultimately it depends on 1) your traffic generation skills 2) your ability to convert that traffic. You can lead a horse to water but....