Some species naturally have a dark 'belly'. There are plenty of site that can help you find out what type of turtle/tortoise you may have found.
Be aware that removing many turtle/tortoise from the wild is illegal. This is because certain species are on the endangered species listing. If the turtle/tortoise is injured, get the turtle/tortoise to a rescue to perhaps rehabilitate and release. This gets a little tricky without your help because they normally live in a very set area so just releasing the turtle/tortoise will leave the turtle/tortoise with no knowledge of its surroundings.