I occasionally get dizzy especially in summer for no reason and i get blurred sight or even worse i get a dark vision and i can't see a thing?

Possible migraine: Your symptoms suggest a possible variant of migraine headaches. Are the vision changes bilateral or just in one eye? Do you have any headache associated with your symptoms? There is no way to actually make any kind of diagnosis here without a thorough exam. I suggest you see your primary care doctor and have a complete eye examination, preferably with an ophthalmologist, who is an MD.