Flower Fairies by Cicely Mary Barker - Black Medick Flower Fairies

"Why are we called 'Black', sister,
When we've yellow flowers!"
"I will show you why, brother:
See these seeds of ours?
Very soon each tiny seed
Will be turning black indeed!"

Black Medick also known as Hop Clover or Yellow Trefoil (medicago lupulina) 

Originally introduced to the United States from Eurasia, black medick has naturalized and is now rather common in lawns, waste places, roadsides, stream valleys, pastures, and fields. Black medic can be a problem weed in lawns. Dense, rounded heads of ten to fifty yellow flowers appear April ­ October. Ripe pods black.  The seeds can remain in the soil for years before growing.

The word Shamrock is related to seamrog, which appears to be generic, being applied to many clovers, the black medick, the pimpernel and the wood sorrel, each one which has been claimed to have been the original Shamrock.

For the old Greeks and Romans, it was a magic plant protecting against snakes and scorpion stings. Regarded as highly sacred, it was largely employed in their religious rites and ceremonies. The grass crown composed of Shamrock leaves and was esteemed a mark of high honour. Spes, or Hope, was a beautiful child standing on tip-toe, a shamrock in her hand, and for the Irish too, the "immortal shamrock," or St. Patrick's cross, is an emblem of hope.

The Druids held the Shamrock in great esteem, because its leaf symbolized the three departments of nature - sea, earth, and heaven.

As a "holy herb" the Shamrock was considered disgusting to witches, and protected against all evil, worn alike by peasant and knight as a potent charm.