Hi, Asif, which plants have you cultured? which growth regulators and medium have you used? There are many possible reason of black young leaf and stunded growth, like high light, not optimal mineral balance etc. You can try dilute medium, or use less growth regulators, less light etc... If you can share with us more details, we can give you more precise suggestions. Good luck!

some times it occurs because of calcium or boron deficiency in tissue culture medium. it depends on your plant your explant, your calcium and boron concentration and pH.