My understanding is that up until 1945(?), taking a blood transfusion was a matter of conscience. Garybuss recently did a thread on this that as late as 1951, you didn't get the Axe for accepting a blood transfusion
Up until 1961 it was a "conscience matter", although still looked own upon, much like smoking up until 1973. Someone with CD-ROM access can look up the QFR discussing this.
There was also a QFR from sometime in the late 50's (again would need help from someone with CD-ROM access) about an "anointed sister" who took a blood transfusion. The question was, "should we bar her from partaking of the emblems this year?" The reply was "No, it's a matter between her & Jehovah, we cannot judge".
I have no idea what caused the major shift between "we cannot judge" to "oh you bet your sweet bippy we can judge, & we will" between the late 50's & 1961.