For a multi-agency approach, you should send the emails individually, because newspapers like to get the exclusive. If you choose to block send the release each will/can pass up on your article as the "other side" can do it. It also has the added benefit that if your article is really interesting, you press release can grow into a feature. And don't forget to reward the publishers with something (say a bottle of wine or a vouchure for your business), they do remember the sender of a gift, so make them your friends rather than just a contact.