Negative psychic energy can refer to one of two things: destructive psychic energy or unbalanced psychic energy.
The first kind of psychic energy has a balanced, clean vibration. 'Negative' in this case means 'empty', 'destructive', 'accepting'. This is simply one end of a complementing spectrum. (On the other end is 'positive psychic energy', which can be described as 'full', 'creative', 'forthcoming'.)
This kind of negative psychic energy might be scary at first, but it is very useful and necessary for a healthy, wholesome state.
The second kind of negative psychic energy refers to a blockade or obstruction in the flow of psychic energy. The vibration of energy can be tainted with many minuscule obstructions, and that's why we can get the feeling of a tainted, soiled kind of energy.
It is not very pleasant, but a psychic has to deal with this kind of energy as well. In fact, a clearly vibrating psychic energy is hard to achieve and even harder to maintain.