Okay so what are they doing about the lack of circulation? Giving you more pain meds will not solve the problem... Sorry but I used to be a nurse so this is a red flag to me.
A black tongue indicates potential tissue failure and necrosis. That can be toxic in and of itself. I know you are freaked but better to be freaked and alive. Than the alternative,
now I'm worried and I don't even know you. This cancer can be highly aggressive. And normal treatment for this is actually a neck dissection ( along with the tongue surgery ) this cancer can seed to the nodes and when it is at the microscopic level it cannot be picked up on a scan. If you have any nodes pop up hightail it back to the drs ASAP. (Lumps in the neck area = potentially cancerous nodes)
Also usually depending on the pathology report they will recommend radiation and chemo. Has any of this been discussed with you?
Best of luck