What you see is quite normal.
The veins under the tongue are bluish black in all individuals.
Do not worry about that.
If the cough
at night is persistent, please visit your doctor to rule out any reflux that is causing the cough. If he has a cold
too, it might be the nasal block and the post nasal drip
that causes the cough at night. Saline nasal drops will be useful in such a case.
Hope my answer helps you.