Have you called the dermatologist you went to and let them know that what they gave you isn't working? I'd recommend starting there since it's important to let a doctor know if what they gave you isn't working.
For blackheads in general, you should wash your face at least twice a day. You will want to use an exfoliate twice a week or so, and you can use a clay mask as well. You can also have them removed by a professional in order to see if that helps. Typically for acne related concerns, I've seen two products work well for people- one is called Proactive and the other is Unblemish by Rodan and Fields- both of those products you should be able to Google and order from their web sites, if you wanted to give one a try.