Dave,
Blyth Library has no church records of BMDs. It has an assortment of Censuses, mainly for the Blyth area. But it has a good selection of C1851 surname indexes for most of old Northumberland. Other "pearls" are Indexes on Ship Owners and Master Mariners, registered on the Tyne, Indexes of Coal Mine owners, Marriage Index for all-Northumberland 1813-1837, Tynemouth Workhouse Registers etc etc.
Church registers for Blyth area are available in Northumberland County Record Office ( Morpeth), and partially in Newcastle City Library and Tyne&Wear County Archives Services( Blandford House, Newcastle , and probably other places, including the appropriate churches.
Michael Dixon