I think it is much easier to just use a unicode character to get the job done. You can look through arrows by googling either Unicode Triangles or Unicode Arrows. Starting with iOS6 Apple changed the character to be an emoji character with a border. To disable the border I add the 0xFE0E Unicode Variation Selector.
NSString *backArrowString = @"\U000025C0\U0000FE0E"; //BLACK LEFT-POINTING TRIANGLE PLUS VARIATION SELECTOR
UIBarButtonItem *backBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:backArrowString style:UIBarButtonItemStylePlain target:nil action:nil];
self.navigationItem.leftButtonItem = backBarButtonItem;
The code block is just for the demo. It would work in any button that accepts an NSString.
For a full list of characters search Google for Unicode character and what you want. Here is the entry for Black Left-Pointing Triangle.