Paul,
If you were to visit the Durham County Record Office you should have a field day as that is where the St Mary's, Blackhill registers are deposited. These records only commence 1857 and prior to that the Catholics of Blackhill/Consett were served from Brooms, Nr Leadgate which is why your direct ancestors married there as follows:
24th November 1851
Patrick Shannon of Berry Edge son of Owen and Bridget Shannon of Co Monaghan
to
Margaret Urwin of Blackhill daughter of Thomas and Elizabeth Urwin of Blackhill
witnessed by: Hugh Finigan and Mary Burns both of Berry Edge.
I'll send you the burial details later: Margaret died in 1873 but Patrick survived up to 1910. Don't be too concerned about ages which can be up to 20 years out.
Good luck,
J.T.A.