Half of my toe nail is brown/black, what could be the cause?

Many things: Many things can cause discoloration of a toe nail. It could be blood resulting from a history of trauma or a fungal infection. I would advise seeing a podiatrist or dermatologist to rule out anything more serious, such as melanoma.

Many possibilities: Some causes of nail discoloration include trauma due to tight shoes, infection, and medication side effects. Get the feet inspected by a professional to determine the cause of the problem, as treatment varies depending on the diagnosis.