Defrosting tuna developed a blue/green sheen? Bad?

[CushionSpiral]

Get fish from a company that freezes at sea. Defrosting the tuna and it starts to get a blue/green sheen and gets stronger the less icy it gets. Not happened before. Assuming this means it’s contaminated with something? Or is this normal? Colours aren’t really coming out as strongly in the photo

It’s normal - not contaminated.

Thanks!
Any ideas why I’ve not noticed it before?

Yes I think normal too

Does it smell?
I’m not sure, did you get it from the same place? How did you defrost it? Fridge or on the work top?

It looks like freezer burn on it.

The colour became stronger the more it defrosted. Left out on the worktop, appeared after only an hour, when parts still frozen.

Quite normal. It’s just light reflecting off muscle fibres, shows it was nice and fresh when it was cut up

Thanks @TonTonMacoute that’s reassuring