The night train is (was) the "Blue Train", not the same.
The Blue Coast / Côte Bleue Line is between Marseille and Martigues. It is a regional line. There used to be day passes for regional lines, available on the blue ticket machines at Marseille Saint Charles, but last time I used them was 2019 and things change often when it comes to French train fares...
In any case, I don't know if there are many places you should get off the train. The train isn't very frequent, so you might as well stay all the way to Martigues, which is interesting to visit (even though the station is a bus ride away from the center). And near Marseille, you could consider a stop Niolon which is a cute hamlet in a cove (also see La Vesse next door), with a good restaurant with great views (Le Mérou) that needs to be booked.