Positive: Sophistication, glamour, security, emotional safety, efficiency, substance.
Negative: Oppression, coldness, menace, heaviness.
Black is all colours, totally absorbed. The psychological implications of that are considerable. It creates protective barriers, as it absorbs all the energy coming towards you, and it enshrouds the personality. Black is essentially an absence of light, since no wavelengths are reflected and it can, therefore be menacing; many people are afraid of the dark.
Positively, it communicates absolute clarity, with no fine nuances. It communicates sophistication and uncompromising excellence and it works particularly well with white. Black creates a perception of weight and seriousness.
It is a myth that black clothes are slimming:
The truth behind the myth is that black is the most recessive colour a matter of not drawing attention to yourself, rather than actually making you look slimmer.