Ahh damn, as usual, I spent ages trying to get this working and the second I make a post, I work it out
Found this snippet on stackoverflow
//Disable writing to screen
BlendState disableColorBufferState = new BlendState();
disableColorBufferState.ColorWriteChannels = ColorWriteChannels.None;
_mask.Material.blendState = disableColorBufferState;
At least I know how to do custom blend states now.