How can I fix my splotchy hair color?

My original hair color is dark brown and I just recently died it black. I used Loreal's Feria 21 Starry Night which is a permanent color. The last time I colored my hair was more than a year ago and it is in a really healthy condition. I have not had any other chemical processes done to my hair and I have never used henna dye. I had someone help me with the application of this product so I wouldn't miss any spots. I also followed the directions carefully but, for some reason the color came out spotty anyway and, now the permanent color is washing out rapidly. The box of color wasn't old as I colored my hair a few days after buying it. I know that sometimes if you buy an old box of color it will turn out spotty. I love how black hair color looks on me so i'd like to know what I can do to fix this mess. It isn't too noticeable but it does bother me. I want to dye my hair a more natural deep black. The deepest black possible. I was wondering if I could just color over top of this old color in a few weeks. Will it fix the spottiness? I also want to know if anyone has any suggestions for a good box of deep black hair color that will last a long time?