The problem may be here

gd does nice cropping to viewports but this one may be too much if $newh<450

if ($newh<450) { 
  $image->copy($resize, 0, (450-$newh ) / 2, 0,0, 600, $newh);
  }
else {
  $image->copy($resize, 0, 0, 0, ($newh - 450) / 2, 600, 450);  
  ]
[download]

Trying to copy from outside the bounds of the src image may have introduced the black, just a guess