A recent injury to the nail can cause a bruise that can look brown, black, purple or red - this should improve with time and grow out with the nail. There are some features to watch out for that can be signs of something more serious.
If you have had no recent injury, this needs further investigation, as it is an unusual place to have moles or marks, and therefore we treat it with more caution if it is new or has changed in colour, size or shape. The concern is a skin cancer called melanoma, and this would usually only affect one nail. You are at slightly higher risk of this being a concerning mark if you are of South-East Asian origin or darker skin, and over 40 years old.
As discussed earlier, if you have had no recent injury, this needs further investigation. You should book an urgent appointment with your doctor, who may ask you more questions and examine you. If they agree that this needs further assessment, they will send you for an appointment with a dermatologist.
If you have a brown mark on your nail you are fit for work.