One of my fingernails is turning black and they all have ridges. ....what could be causing this?

Nail issues: The best thing would be to have it checked out as it could be anything from fungal infection to injury or other disease. If you have ridges it could also be fungal infection but it is difficult to say without looking at it. See your doctor. If it developed acutely it could be an injury but you would have known it since it would be painful to have it bleed and have the blood underneath turn dark.

Hard to say: Ridges wise, if they are vertical then it's unlikely anything concerning. If they are horizontal - Beau's lines - many things can cause these like stress, nutritional issues, or many different systemic diseases. The color change can be from many things too, as simple as a bruise or concerning things like cancer. Before you get too concerned, please seek input from your doctor.