Blue ring around dark brown eyes a vision problem?

Probably not: This is most likely a deposit of lipid term 'corneal arcus'. It is benign and will not affect the vision. If the eye is red, painful, or the vision is changing rapidly, - there are some pathologic changes that can occur in this part of the cornea which will appear like a blue ring. These are rare but will need an ophthalmologist to look at them.