My dog has peed black several times. At times he will not be able to pee at all, then he pees all over. Help!?

We know this causes him pain and have taken him to the vet many times. They have run tests and done scans but found nothing. We have spent thousands on getting him help but am at a loss now. I don't like seeing him in pain but then all the symptoms go away for a month or two at a time. He is a Shetland Sheepdog and is only three.