I guess voles aren’t fond of the scent of blood–which makes sense. Would you want to hang out around someplace that smelled like Hannibal Lecter had been hanging out next door?
Dried blood is a great fertilizer that has been around forever. It’s something that has been discouraging rabbits and the like for a long time. This product just packages it with pepper and bone meal which are also useful to deter animals. Basically you try it before getting out the traps and guns, because a hungry animal will eat from treated areas if they have better food. It’s only the smell blood telling them danger that deters them. I’d put down blood and bone meal over the whole garden for fertilizer if I could afford it.
I use a combination of blood and bone meal in my gardens. IF I apply in regularly (which I haven’t bothered to for two years) it actually keeps the deer away from the hostas, while fertilizing the plants. But it needs to be applied a lot, and its easier to plant things bunnies and deer don’t eat.