Is Misty playing? Trying to get Luca to play/ chase? Can Misty be told off if the behaviour is directed at Luca? Eventually Misty should learn that she is not allowed to chase around Luca.
Can you teach Luca a 'quiet' command? Is Luca barking in an aggressive way or excited playful way .... can't see Misty but aware of movement and may want to join in or even tell Misty off but can't see where she is so barking in a frustrated way?
Our blind Flat Coat was more vocal than other gundogs that we have had. He would even howl like a wolf at certain sounds eg. motorbikes, washing machine when it hit a certain spin cycle, kitchen blender and a certain note on the Coronation Street theme tune
. Taught him a 'quiet' command which made life calmer. Know Luca had a troubled past but will probably start to feel more secure even if he can learn to follow some basic commands.
Difficult to know what kind of barking it is without watching them and how to deal with it would depend on what type of barking it was and why Misty was whizzing past.