Did you call the vet first and tell them you were going to be dropping of a sample so that they could check for blood, or was this something you just decided to do on your own? The problem I see is that the person collecting samples, may not be aware of what they can do test wise. If you had called first and they told you to bring a sample in to be tested for blood, I would demand my $41.00 back. I would probably find a new vet if he refused.
What are you feeding Sammie, and do you ever give pepto bismol, or any type of supplements? Here's some more information:
What Can Cause Black Stools?