I am an artist who have used blogs to promote music. The hard part is getting noticed by the blogs. Just because you send the email doesn't mean they read it. Hip hop blogs get flooded with artist submissions and you have to stand out. It's not easy and it won't take just one submission. You have to be consistent with submitting your music. You also should follow the blog's preferred contact method, so look on the blog to find the best email address or contact method so your music gets seen and heard. I say never submit the same music twice because it may frustrate blog owners and ultimately get your submissions trashed. Anytime you submit music be sure to include all information (song title, song artists, song production, etc.), link to an image and website, and give all contact information (phone, email, website, etc.). This just things to think about when submitting. These little things count big when dealing with getting seen and heard. Anyway, here's a short list of some of the better hip hop blogs out here that take music submissions,
RapRadar.com
2DopeBoyz.com
NahRight.com
StreetDose.com
YouHeardThatNew.com
DJBooth.com
FreeOnSmash.com
AllHipHop.com
DiverseHipHop.com
HipHop-N-More.com