Removing L-shaped elbow underneath a kitchen sink
The nut on the left is threading down onto the black elbow, you need to know this in order to turn it the correct direction. If you think that it is threading up onto the white one then you would be turning it the wrong way and tightening it.

Put your channel lock pliers on the nut so the handles are to the left and move the pliers handles to the right, this should loosen the nut.