How do I remove a black coffee stain?

Some car washes will do your rugs while your car is going through. They use a high pressure sprayer,scrub brush and detergent, then rinse. Seems you could try this at home with a hose. If it's just the rug, have you thought about replacing it? Or perhaps replacing it with a rubber mat that can contain spills and be hosed off?

Not knowing the color of your car rug...I have taken a bleach/water mixture and with a small brush, dabbed the area of the stain. Once it dried..it was a lot easier to color the white area back to same rug color then it was trying to get a dark stain out.

Mix hydrogen peroxide with some blue Dawn dish soap. Spray the stain with this and let it set for about thirty minutes. Scrub with a brush and use a cloth and cool water to rinse. You may have to repeat, put this should remove the stain.

http://www.goodhousekeeping.com/home/cleaning/tips/a16166/stains-coffee-may07/

Club soda may "push" the coffee to the surface. And I found this one

Blot up as much of the coffee as you can. Then, mix one tablespoon of liquid dish soap, one tablespoon of white vinegar and two cups of warm water. Using aclean, white cloth, sponge the stain with the mixture. Apply a little bit at a time, blotting frequently with a dry cloth until the stain disappears.Nov 7, 2017

Spray with carpet shampoo then soak up with clean dry terry cloth rags.

Deb is right, Blue dawn is a miracle in a bottle. That's how they wash birds and other mammals that get caught in oil spills and it is totally safe.

If you want to completely remove that stain, use Dow Scrubbing Bubbles Bathroom Cleaner. It works every time. Spray to cover area, blot with wet rag, repeat a few times and you will love the results.

http://www.scrubbingbubbles.com/Products/Pages/foaming-bathroom-cleaner.aspx