I've put together a lot of pairs of both Blue x BG & BG x BG...
I have found that not every two BGs make nice Blue babies... It took a lot of pairing, spawning, raising, switching mates, repeating... to form pairs that I was happy with... I often found two fish will make ugly Blue babies together, but can make pretty babies with a different mate.
As for seperating... What I've been doing most recently is... about once a week, I remove all the babies I am confident are not Blue... and keep whatever might be Blue. At about 3 weeks old some are ovbiously not going to be Blue, some obviously are, but many are hard to tell. By 6 weeks old it should b e quite obvious.
The difficulty in this approach is when I start moving the fish around and removing some, most of them get a bit pale, making it harder to tell potentially Blue fish with non Blue fish. I've chosen to simply error on the side of caution and only remove those which I am compltely confident are not Blue.
I've had very little problems with this approach other than it's a labor intensive approach. I do find the non Blue babies will out compete teh Blue babies, but the no nBlue babies that are a threat to out compete the Blues are the ones that I am easily able to identify and remove. The questionable fish are the ones that the Blues can compete with.