By 'open pores', I assume you are referring to pores which are quite large in appearance? It would be difficult to say as I can't see your skin for myself, however, it sounds as if you may be referring to pseudo pores.
These are pores which no longer produce sebum. They have probably become enlarged over time due to the presence of black heads blocking them. Eventually they never quite recover their original size. You can't really do anything about these I'm afraid.
If you wear makeup, Clinique do a small range of products which minimize the size of enlarged pores. I have their 'Pore Minimizer Instant Perfector' in my Bridal Make-up kit. You apply it over the affected area and then apply your foundation over this. It helps to refine the skins texture so that the pores are not as noticeable.
As for blackheads? Do you use exfoliation as routine in your skin care regime? This helps to prevent the formation of blackheads.
Also, do you have an oily skin? Are you using the correct products for your skin type?