Yes, you are perfectly correct there. It's what hatcheries do to produce the Sapphire Gems and similar hybrids.
But there are often reasons to do things differently. Some things I can think of:
If you like looking at blue chickens, you may not enjoy having a breeding pen with just blacks and splashes. If you're raising chickens for pleasure, it's reasonable to consider what pleasure you get (or don't get) from your choice of breeding stock, as well as from the chicks they produce.
Most blue breeds (including Blue Australorps) are supposed to have black lacing on the edge of each blue feather. That is only visible on the blues, not the blacks or the splashes. They can also be different shades of blue, ranging from light to dark, and evenly or unevenly colored. Breeding black to splash means you cannot breed the ones with the best lacing, or the correct shade of blue. You have to wait and see their chicks to know if they have the right genes for those traits.
If you are focusing on other traits, like body size and shape or egg production, you may want to choose the birds that are best in those areas, and not much care whether your breeding stock is black, blue, or splash. As long as your flock doesn't become pure black or pure splash, you can always swing in the direction of more blues by how you select breeders in another year.
I think it's common to choose the best rooster, no matter what color he is, and then select hen colors that will work well with his. (i.e. not black/black or splash/splash pairings, if you want the most blues.)
And of course if you set up multiple breeding pens, you can put different combinations in some pens than others.
If you use actual blues in breeding, you get about 50% blue no matter who they are crossed to, and no matter which color is which gender.
Blue x blue gives 50% blue, 25% black, 25% splash
Blue x splash gives 50% blue, 50% splash
Blue x black gives 50% blue, 50% black
So any of those combinations is equally "good," and using a blue rooster means you can use all three colors of hens and still get about 50% blue chicks.
(Personally, if I wanted all blues, I would set up black x splash breeding pens, just like you suggested. But if I was working to improve a breed, I would probably end up with different pairings more often than not.)
