Anydogsbody, I cannot say when the farm was last worked/occupied. I left Blyth more than 50yrs ago. You may be correct in thinking that it was last occupied not much after the 1960s. There are some knowledgeable people on the Northumberland page in RootsChat, so someone will come up with the correct dates re the farm.
If you also have a look in Northumberland section here in Roots, there are several topics re Bebside, Cowpen, and Blyth.