I'm going to be buying a new harmonica soon because my old one is beginning to wear out and a few other reasons. This time I want to go for a good blues sound but I don't know which key that would be. I think I heard that A or C are good keys for blues but I'm not really sure. Thanks for any help.

HI
i think most blues songs are in E, A, G
and what make the blues sound is play low notes.
if you play the harp alone you can buy a low key or a C which make the best blues sound alone, but if you want to play with a band you have to play an A to fit in

if you already had a C key i like to suggest an A harp that you can play in E in 2nd position.

Really, as ORION said, it depends on whether or not you are playing by yourself or with a group. If you are playing with a group, you need to know what key they tend to play in and then get a harp that matches that key in 2nd position. (If they play in E, get an A harp so you can play E in 2nd position. If they play in G, you'll need a C harp, etc.)

If you are playing by yourself, then it doesn't really matter, just go with what sounds good to you. I personally like the key of G for most all my playing, but the key of A is good too. I usually like the lower keys (G & A) over the mid keys (Bb through D) or higher keys (E & F).