Her mother just doesn't have a ton of overlay. Still well within normal. I've had dogs with minimal shading before.
Liver and blue (not blue merle) are dilution genes. Even if there were a dilute dog in her background, it wouldn't partially show in her. Either she would be a dilute or she wouldn't. It's kind of like saying (and there are some silly people out there who believe this is true) that a sable isn't as rich in color because there is a merle parent (sable bred to blue). It either expresses itself or it doesn't. Though a dog can carry the *gene* as a recessive for dilute blue/liver and not show it, but then the dog would still be a normal sable/bi black/bi blue. There would be no difference in color.