To give you an example – I might do a working to improve my chances of getting a job. This will have the natural consequence of the other candidates not getting the job so would effectively cause harm, albeit it indirectly, to my competitors. My key intention is not to deliberately deprive these people of getting the job so I am not performing “black” magic but as I am casting in the full knowledge that I am going to cause negative consequences for other people I am not exactly casting “white magic” either.
I have also seen grey magic used to define practical magic (as opposed to spiritual) – the practical art of manipulating the world. To be honest though I think that’s a slightly bizarre definition as all magic, regardless of colour or type, is essentially an attempt to bring about a change in the world by will and intent.