The nerves that control that exact spot at the inner edge of your shoulder blade don't start in your shoulder. They start in your neck.
They run from the discs in your neck, branch across your upper back, and end right where you feel the knot – and some continue down your arm.
So when a disc in your neck presses on one of those nerves, the pain doesn't show up in your neck. It shows up where the nerve ends. Right there, at your shoulder blade.
(It's why that spot often flares when you turn your head – your shoulder blade doesn't move when you turn your head, but your neck does.)