black liquid from fish

Have you got a trained fishmonger, fillets should be just that

I have just gone off fish!

Perhaps it was its swim bladder? Left in when the fish was gutted?

Thanks for the replies.
Well we've eaten some of it and no problems so far!
I'm in France, and the only place to buy fish now is in the supermarkets. I think they receive the fish already "filleted" , the skin is usually removed too. There used to be a fishmonger but he got pushed out by Carrefour.
I might mention it next time I'm in the shop but doubt if they'll do anything.
It's very much a matter of caveat emptor here.

It might have been just fish blood. I studied the inside of the dogfish at length when i did A level Zoology but the inside of bony fish a bit of a mystery. Unless it was a puffer fish it was probably nothing to fuss about.

Could it have an ink sac from a baby squid that had got attached? In which case, yum. I love black rice or pasta.

I think now that it was fish blood, maybe from the liver.
I must wear my glasses next time!