Is it a federal or Calif state law that a blood test must be administered prior to prescription of heart worm medicine for a dog?

No, it is a standard of care that any reputable veterinarian will adhere to. Giving heartworm medication to a dog that already has heartworms can cause serious medical problems, including the possibility of death. If you find a veterinarian that will give you prescription heart worm medication without first doing a blood test, I would strongly suggest you not take your dog to that veterinarian.