Answer: The perennial geraniums or cranesbills do sometimes spontaneously hybridize in the garden (or possibly while on display at the nursery) so perhaps you have an interesting cross seedling. (It would however be pretty unusual to have a cranesbill bloom over three inches across, let alone in blue.) Assuming the other cranesbills you have are winter hardy in your garden, I would expect this one to be winter hardy as well. I would suggest waiting until spring and then divide it if you want to propagate it. In the meantime, treat it as you do your other cranesbills.
You may be able to identify the parentage of your plant by looking at the leaf shape and texture, summer and fall coloring, and also the plant's overall size and growth habit and match it to plants you have purchased -- or at least have some fun puzzling over it.
There are many different cranesbills in commerce, including a G. pratense Plenum Caeruleum with loose double violet/blue flowers, and a G. pratense Plenum Violaceum with very double deep blue/purple rose-like flowers, as well as G. himalyanense with relatively huge (say 5 cm) blue (but single) blooms for example. Unfortunately I am unable to locate photos of these in a quick search.
Here is a sampling of some of the named types.
Photos of many, in a catalog from a nursery in Wales, just to show some of the vast array.
I'm sorry I can't be more specific for you but I hope this helps. Enjoy!
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