Yes. The hobby is FULL of little quirks. Another example are some breeds with standards that actually call for very separate breeding for male and females. What works for the males makes females that don't even qualify for show and vice versa..
Mario, IMO you could say they are purebred BUT mixed for color. As for birds from blue breeding, you can either say the blacks are well, blacks. Blue as blue. Splash as splash and that will cover it allright. Speckledhen and I were just pointing out using color as a strict breed guide isn't quite that neat as a rule.
Ok here's another example in another species.. German shepherds, most know of them as black and tan in color, right? They come in solid white also.. they still are German shepherds. Not a completely different breed. A cross of black and tan with a white still gives "pure" German shepherds, as a breed.. however as for color the pups will look black and tan except they have the gene for white. By the definition some use in this thread, those very same pups would be considered "mixed" or "mutts".
Except with chickens there can be many different genes involved in making a single color so in their case the crosses and mixes can be wildly variable in color (and for this reason like someone said, it can be a challenge to make birds pure breeding for a COLOR out of the mix)