HI everyone!
In an interesting turn of events a very good friend of ours who used to be a big-wig at GIA saw this thread, and gave me some insight into this incredibly esoteric vein in the diamond world.
Here's some of his observations:

So Daniel, if you're reading this- there's the opinion of someone who is very familiar with natural red diamonds.
He also requested that you show us a photo of this stone under a UV light.
If you are reading this Daniel, email me the photo and I'll post it.

Actually, I saw a round red ruby for sale on another site recently (not a site I normally would check-out... I can't remember how I even landed there... hummm... do you think I may have been casually surfing the net, looking for the OP's "rare blood red diamond... ?"). There are so few rubies out there that when you find them, each is fairly memorable. In this case, I was quite struck by what I saw. The picture for the ruby and the picture for the "rare blood red diamond" stone are so similar that I thought it might be the same stone. I did compare the photos side-by-side, and although they are not copies of the exact same photograph, I concluded that it was very possible that they were each photos of the same stone. It is probably not, however, my point is that I was comparing this "rare blood red diamond" to a ruby, definitely not a diamond. For what it is worth, if it is a natural stone, after ruby, my guess would be rubellite tourmaline.

However, I am not a gem professional. And, photos can be deceiving. I have no standing to argue the gentleman's claim.

Aaaaaaaaaaanyway, wonderful sleuthing, David. Many thanks to your friend for sharing his expertise and to you for passing it along. Except for the terrible, offensive insults, this turns out to be a valuable thread after all, and not for reasons the original poster had in mind. I applaud you, David, for NOT zapping the thread entirely when it got over-the-top ugly or because the premise seems impossible; many other hosts would have hit the delete button immediately.