Blue is just stacking Super Saiyan on top of Super Saiyan God, so I think they should have access to God whenever. I'm not so much bothered by them using SSG as I am at the timing of them using God again. It just feels like shoehorned in fan service.
Yes, since it offers a better balance of stamina and power than standard Super Saiyan Blue, in addition to its boost being relevant enough to SSJB's for the Red/Blue form-switching tactic to work well in battle, as we saw in Vegeta and Black's rematch.