If you deny an applicant a loan because of his credit score you need to send to customer his credit report along with stating the reason of rejection. The customer can look at report and see why his credit score is low. If there is any discrepancy, he can contest. Also, he can make some corrective actions to improve his score like paying off an existing loan.
Given that we are transitioning into a world where algorithms are making decisions to deny you a service, it becomes important that the consumer understands why he was denied a service. The customer needs to be explained in simple terms what were the reasons his service request was denied. The customer should have the opportunity to contest any discrepancy and also should have opportunity to take corrective actions.
Seems there is some progress on this front.
In May 2018, the new European Union General Data Protection Regulation takes effect, including a section giving people a right to get an explanation for automated decisions that affect their lives.