Is this sudden? He might just be darkening to bask. Can you post some photos? You can upload photos here https://www.beardeddragon.org/useruploads/ and post them to your thread using the XIMG button when you reply.
My little one turns pitch black whenever shes in the sun shes normally very tan girl. i took her in to the vet to be safe they said she was completly health but its always nice to make sure that they are