That happens on occasion with clowns of a variety of species - I haven’t looked into the cause much yet, but my understanding is that the blue is dark pigmentation under the white scales (and it may change some with age).
To my knowledge, it’s still relatively uncommon, but people have been working to purposely breed blue into clowns for a few years now:
The holy grail of clownfish breeding used to be an all white morph but since this has been achieved in three species already, breeders have had to set their sights higher, and for an even more exotic…
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