One of my toe nails is black. What can I do? Don't have insurance to go see a doctor

Soak it. It sonds like you may have blood under the nail. If you have had injury to the nail & there is blood under the nail, then soak in hydrogen peroxide to free up the debri under the nail. If no pain the nail will most likely fall off on its own.
Most likely benign. As long as it is not a dark spot that is continuously growing, or irregular in shape - it is most likely not a melanoma, probably a fungal infection which needs no intervention unless there is pain someday or you are diabetic.- you should show it eventually to a doc.