Observation entered because of very dark purplish black of upper surface of leaves.
No interactions present.
The colour of the upper side of leaf is much darker than photograph suggests and no hint of green.
I am aware of the binomial change but even the most up-to-date printed references such as Francis Rose's 'The Wild Flower Key'revised 2006 still uses Ranuculus ficaria and most amateur wild flower enthusiasts would only recognise that name. I think until iSpot put in an automatic binomial update it's OK to use some, not too much, out-of-date binomials.
The plants of my observation are surrounded by dozens of green leafed Lesser Celandines which throws doubt on a nutrient cause. I am inclined to go with the idea thats it is a garden escape of 'Brazen Hussey', or if that cultivar arose from a random mutation and/or if the species hides a recessive gene for it, it would likely pop up from time-to-time anywhere.
I agree with it being very like the cultivar 'Brazen Hussey', which I have seen first hand, will post a photo up so you can compare.
with you both! Christopher Lloyd in 'Colour for Adventurous Gardeners' puts 'Brazen Hussy' in his Black plants chapter, calling the leaves bronze-black - Lloyd's photos show very dark foliage.
It certainly looks like the 'Brazen Hussy' variety I grow in my garden. The original plant was found by Christopher Lloyd growing naturally in a small wood close to Great Dixter so I expect it could pop up from time to time anywhere as BrianW also suggests.
Lat/Lng: 53.3493, -2.1168
OS grid ref: SJ923836
Damp site, shaded by Populus nigra (Vernon Park remnant). Std.green leaved Lesser Celandines in early flower adjacent.