Chicken or quail have black or dark red spots on the meat
It's just clotted blood.

Quail is a far more game-y meat than chicken meaning there is much more muscle and you end up with a lot of dark meat. You get the same with truly free range chickens.

This is nothing to worry about and I have never seen a quail which doesn't look like that.

Source: My mum had a poultry farm.