but it is not officially GDR3 its developer preview this s not a proper officially update for Lumia's so Nokia Promised GDR3 updates ill push in Mid January for all lumia phones but yet could not exactly announced/
I think you're misunderstanding here.
Developer preview provides you with GDR3 and its features are directly from Microsoft.
However, since you're using a Lumia, Nokia tried to add some more features to that GDR3 and call it Black update. Which mean, the Black itself comes along with GDR3 with a few more features from Nokia. If you're using developer preview, then you update to Black, the differences you can see are just Nokia's features.
So in summary, if you didn't update GDR3 using developer preview, then you would see many more changes in Black (compare to GDR2 or Amber). Otherwise, it's just adding some more features as unnatixlr8 said