Hi there, hope you are enjoying BYC!
Depending on the breeding.. Blue only breeds true 50%.
If you (they) are breeding blue to blue... you will get 50% blue, 25% black, and 25% splash.
Only by breeding splash to black can you guarantee 100% blue birds... but then they will be a variety of shades of blue... some darker, some lighter, and no consistency. Are you in the US? Which hatchery are they from?
I would suspect they are the same breed with different phenotype's (genes expressing visually)... the one with light toes.. maybe splash for example. Once a breeder is familiar with the different chick phenotype's their birds produce they may be able to discern which are keepers verses cull right at hatch... as certain things will grow out certain ways.
So the case here is either the grey will be blue and the lighter one will be splash... or they will BOTH be blue but different shades... According to what little understanding I do have after ALL these years of learning and practicing.
It would be so cool if you reported back with pics of the final outcome!