People get so hung up on labels. Everything has to be a breed, something sold under a marketing name, or a cross with a nickname. Whether or not you should breed the depends on your goals and what you want.Just to confirm they are not considered an Australorp? Should I not breed them then?
I don't know where you are located. A breed is defined by a country's poultry association. Different countries can have different requirements. A good example is the Araucana. The appearance of the British Araucana can be a lot different than the American Araucana. I'm not talking about feather color, I'm talking about body shape and even whether they have tail feathers or not. There are a lot of different things that go into what makes a breed: eye color, comb type, feathering, ear lobe color, leg color, color of eggs they lay, body size, and body conformation. A good judge can probably tell breed by looking at a black and white silhouette of the chicken if it is a good representative of the breed. Body conformation is a huge part of a breed and that can be different in different countries.
Along with the requirements of the breed you get approved colors/patterns. Some breeders do not consider color as part of what makes a breed. If you are showing them then colors become important. If you are trying to get a new color or pattern approved you need everything else to be right for it to be that breed. This can get confusing. To many people color is a very important part of the breed, to others not so much.
I found this little blurb on Australorp.
Here in the US the recognized color is black. However in Australia they recognize the black, white and blue varieties. You can even find this breed in buff, splash, wheaten faced and golden.
In Australia they have approved three different colors but they have other people breeding Australorp in different colors. I don't know why, they just are. In the US the only approved color is black but at least one hatchery sells Blue Australorp. The one I'm thinking of said they imported them from Australia. I don't know how well they meet the US SOP (Standard Of Perfection) breed requirements or how closely they meet the Australian breed requirements.
As I said, whether or not you should breed them depends on your goals. If the breed requirements are important to you then you should find your country's breed requirements and breed them if you believe they will give you chickens that meet those requirements. You may want to show them, sell eggs or chicks as representatives of that breed, or it just may be important to you. If you just want chickens and those meet your requirements then breed shouldn't necessarily matter. They will still lay eggs, taste like chicken, and do all the things that chickens do. It just depends on your goals and wants.