It's probably some sneak carried genes. Generally blues will just produce the normal BBS, they'll generally not have too many carried genes cause most people breed BBS and don't outcross ime. But seeing as yours also carry white, there's a decent enough chance that they aren't homozygous for a...
Can you post pictures of the parents? you're absolutely positive those hens had no other access to another rooster for 1-2 months prior to you collecting the eggs?