Music blogger and publicist here, weighing in. There's so much I could say about it, but the short version is, be prepared to spend a lot of hours on this. It takes a lot of work and research to find those blogs, and then even more to build those relationships with bloggers, so that you don't become just another email in their inbox that they ignore. This is why most people hire a publicist.
However, that being said, a good way to find a blog that's likely to be into your music, is to identify which major artists you sound like (be real with yourself here) as well as some indie artists that you sound like, who are starting to get traction. The idea with the indie artists is that if you can find an artist that's still unsigned and gaining traction, but perhaps a bit ahead of you in terms of their musical resume, then if you find a blog that covers them, you can be more confident they'll also cover you, because it shows they're interested in covering emerging artists just as much as nationally recognized ones. From there, it's as simple as Googling the heck out of these blogs/artists until you can start to find blogs that they've been featured on, that look like a fit for you as well.
Another technique is good old fashioned digging. HypeMachine features a LOT of blogs (and also allows you to search by artist, meaning you can also use the above technique here), so once you have a stack of blogs to start going through, just make sure they're actually going to be interested in your music by checking out what they've covered in the last few months, and getting to know their blog. Remember, you're asking them to take time out of their day to check out your music, so if you're not willing to spend 10 minutes getting really familiar with their blog and what they do, why should they spend that time getting to know you and what you do?
I could talk about this ALL day, but hopefully this offers you a good start!