I've just seen a blackbird eating a goldfish from my garden pond. I don't know if it was caught live (iooked fresh but wasn't moving). Fish was ~5cm long. Bird hopped away, tried a few times to swallow it whole, then resorted to pecking. IKnow

No it isn’t well known behaviour.  but The main reason why is cause it didn’t eveolve the right beak to eat fish but developed the right beak to Eat insects. a Blackbird wouldn’t be able to successfully swallow fish cause the fish would be slippery. Birds that have evolved to hunt fish have developed things to ensure the fish can be gripped well enough and can be  swallowed whole even though there slippery. 

I'll add it seems very strange a blackbird would enter water to get a fish.

That would normally be the actions of a large wader like a heron or egret, or a duck swimming, neither of which a blackbirds would do!

If, and only if, it was a blackbird eating a goldfish, I would suspect the fish had been taken out of the water by another creature (a bird or animal), and the blackbird was grabbing the opportunity.

Yes.  Blackbirds arent normally  interested in catching fish. Birds develop the Anatomy, and hunting behaviours that help them survive over time. With Wild animals evolution favoured the anatomy and hunting styles that helped them hunt and survive over the ones that didnt  support there diet, suitable habitat and survival. Some species are better adapted to a habitat where You can find things like fish for example so evolved the anatomy, looks and hunting styles that give them the advantage over the food they need to catch.  Since some other birds like the Blackbird are more adapted to hunting for insects a herons hunting style wouldn’t be helpful for a Blackbird cause The Blackbird wouldnt be able to use the hunting style required to catch fish so it wouldnt help there survival. Cause of that they wouldn’t normally go in the water to catch fish so I know what you mean  it does seem strange.