ViewController.m

// Create a new layer which is the width of the device and with a heigh
// of 0.5px.
CALayer *topBorder = [CALayer layer];
topBorder.frame = CGRectMake(0.0f, 0.0f, self.view.frame.size.width, 0.5f);

// Set the background colour of the new layer to the colour you wish to
// use for the border.
topBorder.backgroundColor = [[UIColor blueColor] CGColor];

// Add the later to the tab bar's existing layer
[self.tabBar.layer addSublayer:topBorder];