Black Medic has yellow flowers actually. The seeds when dry become black and I think that is why it got the name. These plants can get really big and grow low to the ground. They tend to show up in thin lawns or nutrient deficient lawns. They have a long taproot, which needs to be removed or killed entirely or it will come back.It can be confused with some similar looking weeds such as Hop Trefoil and Lesser Hop Trefoil but Black Medic has a hairier stem and leaves as you can see in the picture below. Unlike other weeds we have featured here Black Medic, it only produces a few thousand seeds per year and not 20,000-40,000 like Kochia and Dandelions. Like all weeds, best to pull or kill them before they start to flower and produce seeds.
More info and source: https://extension.usu.edu/weedguides/files/uploads/fabaceae.pdf