Some would define white magic as being good and black or dark magic as being either evil or negative to say the least.
White magic is, generally speaking, used for "good" purposes whereas black or dark is usually relegated to evil or negative purposes.
Good is anything that is done genuinely and unselfishly to help, encourage and engender wellbeing and care for others. It is done with good intention. Whereas evil or negative is to the contrary and includes actions that manipulate anothers will - regardless of whether or not the outcome is positive.
Another school of thought is that the magic itself is neither white nor black. That magic is simply a tool that we use, just like a hammer or a pen, and that it is the intention within a persons own heart that is white or black.
Flutterbit