Is it possible that the blackout of vision for a few seconds is due to heart blockage?

Not likely.: Possible but not likely. Transient obscuration of vision is more likely due to a transient drop in blood pressure which could be from an electrical problem in the heart but not generally an intrinsic plumbing or blood flow problem unless of course the symptoms are strictly exertional. Other concerns are problems with vascular tone including side effects of meds and dehydration