After some hilarious googling (I had no idea folk singers were such popular heroes and heroines in romance novels) I found a possibility.

The Sea Waif, 1967, by Anne Weale (cover shows heroine with black hair), Harlequin Romance #1123:

http://www.fantasticfiction.co.uk/w/anne-weale/sea-waif.htm

"The glamorous, sucessful folk singer Sara Winchester was trying to make a fundamental decision - whether or not to marry a charming, eminently suitable man who loved her deeply. But still she hesitated. Why? It was for a reasonthat took her back six years - to another world, the world in which she had been so happy with her guardian, Jonathan Logan. For whatever might happen in the future, Sara knew that between her and Jonathan had been forged a link that would never be broken..."