I had a stain on my teeth.so my dentist suggested that I use the tray with bleaching gel.but the stain turn darker.my dentist said it was the bleaching gel that caused the stain to turn dark. I have not received a direct answer.

And your question is ? It is possible it reacted to something in the gel.

What kind of stain did you have? Is it inside the tooth? If the whitening gel didn't help it you can always have your dentist buff it down slightly and place a filling that matches the rest of the tooth over it.