What is causing this/what is this?
 

My orchids new growth has a large black spot on the side of it. What causes this/what is this?

thanks

Could be that water got in between the tender new leaves and started rot. Or it could be mechanical damage.
Keep an eye on it if it starts to get larger you may want to cut that portion off. You may want to try cinnamon on it which is a natural fungicide.

Dittos Ahab. The plant could have been damaged from rubbing the potting media as the new growth was emerging. Nevertheless, it should be treated as a fungal problem and sprayed with peroxide and dusted with cinnamon.