Decay under composite temporary veneers? (Photo)

I had some composite temporary veneers applied 2 weeks ago. They were turning sort of gray and they were removed today. Underneath, my teeth were almost completely black. My dentist said it was only on the surface since she was able to remove the black fairly easily. She said it was because my gums were bleeding when the composites were applied. How can I be sure it wasn't due to some other reason such as decay, or be that way with my final veneers? I don't want to continue until I know..