That's the periwinkle Vinca minor in the image above. There are some houses on the other side of the creek with steep front yards, and some of them are planted to periwinkle. As I was driving by the other day the mass of blue color caught my eye. I came back the next day to get some photos.
Here and there I've seen periwinkle used as a companion plant for daffodils - it's an appealing combination, especially with white-flowered daffodils.