Hi,
"oh, good God"
I'm not so sure I'd say categorically that this is always blasphemy, if you define blasphemy as something like 'treating a sacred thing with a lack of reverence'.
Take an extreme case. If my car starts to skid off the road towards a tree and I say "Oh, good God", you could make a case that I am instinctively appealing to God for help. The same may be true when I use the phrase in other less serious situations.
Best wishes, Clive