Necrosis or Hematoma After Juvederm Lip Injections?

 This sounds pretty classic for a hematoma, the darkness from a intravascular injection which could lead to necrosis would be much later than you experienced.

If you had necrosis you would have had a white blanching first as the area would not have enjoyed a blood supply. This would last for while (at least an hour) and then the region would darken and turn black as the tissue began to die.
It sounds like one of your blood vessels was hit, perhaps a large one, but it does not seem to me like tissue necrosis. It also appears like you had a physician available to monitor the situation. If necrosis was developing your physician would have injected hyalurnidase. This dissolves the filler.
The lips do not seem to be an area prone to this kind of problem. The lips are quite vascular. Most cases of necrosis which are reported occur in the glabellar region and nasal tip. Sometimes the forehead and cheeks can also develop this somewhat terrifying complicatioin.