Once again, WhiteRav3n is quite right. Auras are a reflexion of what is inside and you need to look into yourself to gain some clarity on the situation, find peace, and possibly cleanse yourself to remove negative energy
However, I don't always see black (or grey) as a negative colour. Firstly, to me, grey symbolizes balance. In the case of black, yes it can signify negativity, lack of forgiveness, pessimism, depression and those things.
Generally, the brighter and clearer the colors in the aura, the more positive and healthy that person is on various levels. Murky, dull colors usually denote lower vibrations, blocks, issues, negativity, etc. This does not mean that black is "bad."
A black aura can represent mystery, change, power,transcendence of ordinary reality, dignity and new potential. Black can also be consciously employed to uncover whatever is hidden, which can be helpful. Also, in fact some people say that a black aura is a sign of people who are great with energy work, however this is based on opinion.
In the end, black can be a colour of both negativity and positivity.