I don't know about a blood test, but you could have an endoscopy done. Probably would cost more than its worth though, and you would have to find a vet knowledgeable enough to do it as well.
I had a fellow here taking blood samples and toe nail clippings from my then large "creep" of Russian (Agrionemys horsfieldii) tortoises. He told me that there hasn't been any DNA studies done to know the difference between male/female yet.
It would be DNA, I think, a friend that was a research scientist at St. Jude here in Memphis was one of the first ones that used to sex birds by a blood feather ,he has a lab and would sex birds from all over the world but that was so many years ago, I would love to be able to buy a sexed pair of hatchlings for future breeders