Blue Trout (Oncorhynchus mykiss coerulus)
Current record: 19lb 12oz
Water: Unknown
Blue trout aren't a distinct species, they're a colour variant of farmed rainbow trout (occasionally brown trout), bred for a striking blue-grey sheen rather than the usual pink-spotted rainbow colouring. The colouring comes from selective breeding at the fish farm, not from diet or water conditions, so a blue trout stays blue for life.
They're raised specifically for stocking into UK stillwaters and put-and-take fisheries, often alongside golden trout and standard rainbows, giving anglers a bit of variety and a genuine eye-catcher when one comes over the net. Fight-wise, they're every bit as strong as a standard rainbow, expect hard, fast runs and the odd jump, and most venues stock them at a decent average size specifically because they're popular as a specimen target.
Since they're farm-raised and stocked rather than self-sustaining, you'll only find blue trout in waters that actively put them in, check ahead if you're chasing one specifically, rather than assuming they'll turn up on any trout water.