[Endoperez]

There's no way to automatically pool all blood slaves of all blood hunters everywhere while letting the priests keep sacrificing.

You can, however, press 'Z' to pool all blood slaves the chosen commander(s) have. So you have to do it yourself, but it's pretty easy to do if you group your hunters together. You go to a province, choose one member of the hunter-group, and press 'Z'. That's as good as it gets AFAIK.