It's tough to say, because blue Australorps aren't a Standard variety like the black ones. But if the undersides of her feet are white, than she's probably what the seller said she is. The only other variety she could be a is a blue Orpington, really, and she looks too small for that.
She is also a little young, so it will be easier to compare her to other Australorps when she's full grown.
If you are concerned about her, then you could use a black Australorp female with your blue cockerel, which would give you 50% blue and 50% black chicks. Mating blue to blue will give you 50% blue chicks, too, with 25% black and 25% splash.