Can playing blindfold chess be learned or is it a natural skill? Any blindfold players here? Just wondering if blindfold chess is a skill that can be learned by amateurs like me or a natural skill by chess GM's, IM's, etc.
In essence, you are asking- 


  Can memory (or other brain functions) be improved or is it a natural trait?


The answer is yes to both. While it is true that one's memory can be improved, it is also commonly observed that some people are more "gifted" when it comes to memory (and other brain functions) than others. 

To play blindfold chess, you need three things - 


A decent memory
A good ability to visualize
A decent level of chess playing strength (if you want to play decent chess i.e. or one could simply have a fantastic memory and make legal yet nonsensical moves). 


Memory 

I never "practiced" blindfold chess to learn it. I simply observed that as soon as my chess level got to FIDE 2000 level (for example), I could easily replay moves in my head and think "blindly". I don't have a great memory. I suppose my memory is average or slightly above average, but chess has surely helped to improve it to some extent. 

Visualization

The way I do it is by visualizing a 2D board in my head (like a computer screen). The board doesn't have to be crystal clear (it's often hazy), but as long as you can "see" the pieces in their places, it's fine. I suppose that a chess player's ability to visualize increases the more chess he/she plays. This is normal brain development. The more you exercise a muscle, the better it gets. So I would suggest you practice visualization to get better at it. 

Playing Strength

The real point of blind chess is to play chess blindly and yet play it well. So playing strength is indispensable or else there is no point in playing blind chess. If you play blind chess badly, you only prove you have a good memory and visualization power and nothing more. So, as an amateur, I would say your top priority is to improve your playing strength. Work on that and your memory and visualization will automatically improve. 

All the best!