Are you able to post a picture? Is the black line in/on the nail itself or is it on the skin?
The line is in the nail.
Thanks. It is a line or more of a diffuse coloring? Definitely black or more brown?
Does the toe seem to bother her or seem swollen? Is that nail growing faster or thicker than the other nails?
Melanoma of the toe/nail tends to be pretty invasive and ulcerative. I've never seen it as just a stripe down an otherwise normal nail. Sometimes yeast infections can cause a dark brown staining on the nail that doesn't scrape off and I've seen that on only 1 nail.
If it is melanoma, than amputation of that digit would be the first thing done. There is also a melanoma vaccine that an oncologist would likely recommend.
Here is a good website with more info on melanomas: http://www.VeterinaryPartner.com/Content.plx?P=A&A=2941
The line is a clearly defined line. Not diffused. The nail looks otherwise fine and the foot otherwise is fine too.
Without seeing it in person, but based on your description, I'm guessing it is nothing serious. Could be due to trauma to the nail itself. But I would go ahead and have it checked out.
Do you have any other questions or comments?