I don’t know why the line is blurry, what makes you say that? If you take a dualistic view it’s black vs white, LHP vs RHP, dark vs light.
I think its a matter of semantics and etymology. Black or sinister aka LHP magick generally just means self serving as opposed to selfless. It can also mean “evil” because some self serving magick is going to be baneful.
It has nothing to do with entities. Magick is just a tool, and a tool is neutral: it’s the intention of the practitioner that matters. Pray to “god” for healing for another: white magick, pray to that same entity for harm: that’s a curse so it’s black magick.