it probably has gotten better. i am assuming that nail is no longer sore swollen or painful correct? this is called a subungual hematoma -- a pocket of blood that formed under the nail from the injury. in most cases this blood will eventually be reabsorbed by the body and/or the nail will slowly grow out taking this black spot with it. in your case the black spot is likely dried blood on the nail bed that cannot be reabsorbed or is being s l o w l y reabsorbed. this innocent cosmetic imperfection can last quite a while so you may have to cover it up with nail polish (i am guessing from your screen name that you are female). should you ever have a similar crush injury of your nails in the future the pocket can be removed by a tiny bore hole in the nail (surprisingly painless) when it is fresh. this will improve the pain and reduce this slow healing process that you are experiencing now. the nail can be surgically removed but that may be a bit drastic.
i banged my thumb months ago and have had a black bruise under the nail since then. why has this not gotten better?