I remember a children’s book about some sort of creature or monster who lived underwater in a castle or palace. He is sort of humanoid/frog cross, is described as having a heart as black as coal, and falls in love with a girl (maybe a princess). He kidnaps her to his underwater palace, and she is eventually rescued by her human love interest. It was a book my grandma bought from her library – my guess is that it was probably published in the 60s or 70s, and I think it was watercolored. I also feel like the color purple was heavily featured in it, but that could be wrong.