Thanks for the information. Sorry about the wait. A permanent erection is a serious concern in a dog. The condition is called paraphimosis, and it usually requires emergency treatment. Cool water soaks and application of a dextrose containing solution is necessary to reduce swelling. The penis is then lubricated to place back into the prepuce. Sometimes a urinary catheter may be needed to keep the urethra patent. The concern here is that a dog with an erection more than 1 day is at serious risk of tissue necrosis of the penis. Usually, at that point, amputation of the penis is necessary. I am concerned that the darkened areas you are seeing is necrosis. My recommendation would to be to get him to your nearest emergency vet immediately to get him evaluated. The website below has more information for you. Please let me know if you have any other questions. Hope this helps.