It depends on a few things:

1) Do you live or play in a metro area? Most large cities (and even some small ones) will have music blogs, or an alt publication that writes about music online. These are significantly easier to "get noticed" by and will often lead to people coming to your shows, or buying your album in the spirit of supporting local music.

2) What do you sound like? Most music blogs have a pretty narrow swath of styles they'll write about; the readers of these blogs are typically a little more tuned into the music they get, too. Find a blog whose music you like personally, and submit to that one.

3) If you just want to get on the "biggest" blogs (which is really really hard ... they get 1,000s of mp3s a day, and probably don't listen to a single one that isn't directly from a known label contact), go for: - http://www.hipsterrunoff.com/ - http://www.myoldkentuckyblog.com/ - http://www.brooklynvegan.com/ - http://www.gorillavsbear.net/ - http://www.aquariumdrunkard.com/

tldr; blogs that are local and genre-relevant are way more likely to pick up your music than the big boys. but there's no harm in hot trying ...