From what I have seen of the blues, mine included, in the deeper end there are a lot of shadows that can look a lot like discoloration when in low light, changing when viewed at different angles. But, when bright sun or bright artificial light, it all looks uniform. Seems a real pigment difference would be there at any time, whereas difference in the throw of the light can be affected by other things. So, looks about normal to me unless that lighter area stays that way always.