How can I improve the look of black triangles between new veneers? (Photo)

I don't think you are being fobbed off, but I do think you should seek out a second opinion.  True, he cannot get rid of the spaces without changing the shape to more square.  Also true, all veneers should ideally replaced if you do not like the color.  Perhaps a cosmetic waxup demonstrating a new shape could let you both see if the tooth shape changes would be acceptable. 

I am so sorry about the money you have spent having seven veneers placed and than not being happy with the outcome! I think the doc should have done a wax up so you could have seen what your smile would look like before he had them made.  You would have had the option to see different shapes of the teeth with veneers, making sure the triangles did not show. I would ask him to redo them, and work with you so you have less black area between the teeth. You can always have a little derma filler injected into this area but it will only last for 6 months at best but it will increase the gum area closing the black triangle. He could offer to do this  at no charge rather then redo your Veneers.  If he is a good cosmetic dentist, I would make sure I was happy  with the my smile. Good luck and make sure you work to achieve the smile you were looking for ....you paid a lot of money out and you and he both need to be happy with the result!