Blue Point was not a subsidiary of Snap-On. All the tools were made in the same place. The sockets and drive tools were exclusively branded Snap-On, the wrenches (open-end, box end, etc) were all branded Blue Point (or Blue-Point, Blue-Points). Even after the 1948 change that Snapmom references, when the company also started branding some wrenches as Snap-On, everything was still coming out of the same factory, with no changes in moulds, dies, process, (or quality). These were co-equal brands for a long time.
I can't and won't speak to when it changed, but I would guess the 60's. Snapmom?
EDIT: Quickdrawmcgraw here didn't read your post close enough, pfbz. Sorry for being redundant. You and Snapmom are on it...