What causes a blue freckle/sunspot?

Blue freckle?: Personally, I have never heard of a blue freckle. Sun spots and other actinic damage are caused by the sun and exposure to uv radiation.

Get it off: A blue freckle, as you call it, is most likely a blue nevus. These are very darkly pigmented skin lesions that are so dark they appear almost blue/black. You should see a plastic surgeon with a special interest in skin or a dermatologist as this should be removed. The worse case scenario could be that this is a melanoma which can have dire consequences. Be well and get it checked.