Just from the picture, it does look like four lavenders to me ... but then I am not an expert ... it also looks like the blue one in the background is in the same pen ...
That looks like a blue (or possibly a black, having a hard time telling on the monitor) in the background-I wonder if that bird is responsible for the black chicks? Do they keep them separate?
I agree with your assessment-it sounds like one of them must be a blue. Lavender breeds true and an all lavender breeding should not yield black chicks. I ordered some lavenders a while back (eggs) and ended up with a blue. It took me a while to figure it out, and I wouldn't have except that...