I agree with what Donna says, but it does seem like an unusually short period of time for shoes to last. If you were having foot or joint pain, my first thought would be that you've lost the cushioning in your shoes, but since the problem is a blister, have you checked to make sure your socks are ok and haven't worn thin? Also, what kind of shoes have you been wearing during the day? If they rub your feet at all and your workout shoes are a little tight in the same spot, you might get a blister even if both pairs of shoes are reasonably comfortable separately. Finally, is there a reason your feet might be a little larger or smaller than normal (walking/standing a lot, water retention, weather)? If so, it might help to lace your shoes tighter or looser, or wear different socks. Of course it could still be the shoes, these are just some other things I thought of.
Audrey