I don't really know.
The mottlng gene does cause a chicken to get more white as they get older, which would perfectly explain why she started almost all blue and ended up white. But the in-between photos do not look quite right, because of the way mottling usually shows on the feathers (white tip first, and I would expect quite a bit of white in the wing primaries). Also, having her completely white at 9 months old is much faster than I would expect if the mottling gene was responsible.
There are several alleles of the mottling gene (several different forms, that are all the "same" in their inheritance and general results, but not identical: slightly different mutations). I suppose there could be one that causes the effect that chicken has, but I really do not know for sure.
Regarding the title, why does it say Brahma Cochin? Based on the pictures I would think she is just a Cochin with no Brahma involved.