I think she's quite beautiful (and I'm a hardcore Wellie fan!). If she's sweet, breed doesn't matter!those are for sure wellie looking babys but i agree shes a cross of some kind..maybe a black australorp cross..unless you plan on showing her i wouldnt give it another thought tho..and in my opinion its the black feathered head that makes me think its a mixed breed..
i guess some breeders can give us birds with hatchery quality..you really never no what ya have till ya raise them ..i would think if the person you bought these birds from was a real breeder of wellies then he wouldnt have any mixed breeds in his breeding pens..altho your other birds all look good and this bird you say has taken a shine to you and you to her ..like i said unless you plan on showing them i wouldnt get to worked up over it..but i would let him[the breeder] know tho..
(I'm facing the same with two sweet pullets we just picked up- labeled as Rocks, but white skin and pale legs make me think Marans. But they're so sweet it doesn't really matter what breed they are!)