In the case if Birmingham Sunday by Richard Fariña; I think the men asking about blackberries in the sea are making the statement that blacks and whites together are unnatural. An exasperated protagonist may then be asking: "how many people are out there who would make such a statement?" by asking of dark ships in the forest.

The lyric, no matter the form is an ear-worm and can mean almost anything depending on the context and the background of the audience. Perhaps the latter much more than the former.

I've now been thinking about it for almost 30 minutes after listening to the song.