I imagine detecting black magic is a lot like, say, detecting if someone has a gun. If they're not using it, and you aren't looking for it, you're not going to see it.
If they're not using it, but you're searching them for it, you can find it if you know what you're doing.
If they are using it right now, it's big and obvious to anyone who has the senses to detect it.
And if they just used it, there will be lingering signs on them (the smell of gunpowder, etc.)
And places it's been used recently will retain evidence of it for a little while (bulletholes, casings, bodies).