What you're seeing is not 'blockwork' but 'formwork' impressions.
Someone correct me if I'm wrong, but as far as I can tell the original (sans elastomeric paint) exterior surface was created by in situ gunite against formwork. Because it was sprayed onto the backs of the forms and built up, the forms could be lighter weight, and consequently weren't flexed or pressurized by a plastic mix -- hence the 4' x 8' sheetmarks. Given the inverted conic form of the walls, conventional forms and construction would have had to have been massively overbuilt buttressed affairs to hold back the lateral pre-cure forces.
On a related note, chips of the gunite were removed from surface cracks and made into jewelry during the restoration.