In continuing the discussion on out crosses with guppies, one of the crosses which I have carried out is blues with HB(Half-Black) blues or other way around, HB blues are known to have fertility issues and they can become weak over a period of time when they are constantly in-bred or even line bred, so having a line of blues helps in this way, crossing the blues to HB blues will bring back the vigor and lost qualities in couple of generations. Again as with any out-cross, you have to know your fish, where the HB gene is, is it on male or female with respect to HB blues, is the size on your blues on males or females, again the cross would depend on what your end goal is, is it color which you are trying to improve or size or both or some other feature in the fish. Also it would be lot easier if you are having your own line of blues to cross, other than going out to get from another breeder as you would have start guessing on genetics of that blues then unless the breeder is forthcoming in information regarding their line of blues.
Below is a line of my blues, these fish are little over 2 months. I also maintain another line of blues to keep blues going, topic for another day
Below is one of my HB blue line