The Blackest Crow is a lovely song. Also, If I Were a Featherbed (John McCutcheon). I've done both for weddings. Both have verses that can be bittersweet, but thouse can be altered or left out.

The blackest crow that ever flew would surely turn to white, if ever I prove false to you bright day would turn to night. Bright day would turn to night my love, the elements would mourn, if ever I prover false to you, the seas would rage and burn. (1st verse)

If I were a featherbed, in your house so fine, I'd hold you in my arms each night and keep you warm in the wintertime. If I were an old wool rug sitting in your front hall, I'd tickl your feet and make you laugh if you stepped on me at all. (1st verse)