Hi Nanas Girl,
Hopefully you have been able to get her to the doctors by now and probably the blisters have cleared up by not, but just in case, here is a possibility. Has she taken Tylonal before? If yes has she taken that strength before? It is suppose to be one of the safest pain killers, but for me that is not true. I am allergic to it. My reaction is much more serious, but if she is very elderly, it could have tiny ruptures in some blood vessels. That would account for the black appearance and the tiny blisters.
I don't have the exact same type of cancer as your grandmother, I do have adenocarcinoma of the minor salivary gland that was located in the hard palate in the roof of my mouth. I have lived with it for 9 years. While the survival rate is only 54% after 15 years out from diagnosis, those odds are much better than when I was first diagnosed.
Please try and get her back to the doctor. While they can not cure her, there are things that can help prolong her life. Another thing that may sound silly to you is her attitude. They have proven with various studies that terminal cancer patients with a positive attitude tend to servive 2 years longer than those who don't. While I have recently undergone surgery for a recurrance of the cancer which cuts my survival rate down to around 25%, I am determined to make it at least another 10 years to my 65th birthday :) By then they should come up with something new that will help me make it to 75 :)
Anyway, I hope you and your grandmother are doing well and wish you all the best.