The Penpal of Darkness

Question

I used to play tennis,
but not anymore
A pen pal of darkness,
though you hate our speech
A friend of a friend
of a bar and a horse
In children's games,
I help with the layout for each

I am down to Earth
I wouldn't be a great dam,
But for what it's worth
Can you guess what I am?

Answer 1

I think you are

Chalk

I used to play tennis,
but not anymore

Chalk used to be used to mark Tennis Courts, now I believe paint is far more common.

A pen pal of darkness,
though you hate our speech

Used to write on blackboards, sometimes squeaking most unpleasantly

A friend of a friend
of a bar and a horse

Used on hands in Gymnastics, including several events involving Bars, and the Pommel Horse

In children's games,
I help with the layout for each

Chalk used to mark lines for a bunch of kid's games, including Hopscotch and FourSquare

I am down to Earth
I wouldn't be a great dam,

Mineral from rock or soil, but dissolves in water.