When somebody is watching the TV, I stand between him and the TV, and make him unable to see the screen.
Do I block him or the TV?

Thank you.

The natural thing to say is
eg Am I in your way?

Thanks, Clive. I have learned something new.
Students: Are you brave enough to let our tutors analyse your pronunciation?
You are blocking the TV. Maybe only Americans say that.
OK. Thank you very much.

I can't see the moon, because the high trees are in my way.

Is it natural?

The trees are in the way. I think that for something to be in your way it has to be stopping you from going somewhere.
Students: We have free audio pronunciation exercises.
Thank you