The blue isbar are supposed to lay green eggs. I have the offspring of a green layer (Havoc) who lays brown eggs, so it's possible to get a brown layer out of a green egg (only 1 copy of the blue shell gene). Dunno if it's possible to get a white egg layer out of that: parent R has brown sprayer with white shell and parent H has blue shell without brown sprayer seems somewhat plausible for her egg mother.... in which case she could have come from a white egg...... and lay blue or green eggs (shell color from the father R).
Disclaimer: this is getting into some wild speculation, but does fit the way she looks....and that the only white eggs came from the tolbunt supplier.