PDLM - I am assuming you are not a dog lover otherwise you wouldn't be wondering why us dog lovers would ever think of obtaining blood or anything else from dogs that are about to be slaughtered for food.
In any case, as already mentioned by someone earlier, the blood may be contaminated in some way. You would have to pay for a blood check/count to see if there was anything wrong with the blood in order to use it.
My dog was diagnosed with tick fever last christmas and he nearly died, they did an emergency blood transfusion as the last resort and luckily he survived. Tick fever is lethal when the parasit gets into the blood stream and kills the healthy red cells so the dog becomes aneamic and can't regenerate cells to heal itself. Now imagine that Delia's dog got a transfusion with tainted blood suck as tick fever...
It just doesn't make sense in any way to use blood obtained that way.