If it was groundcover, it might be Crown Vetch, which is more lavender in color. It is pretty, but it's considered invasive. Sometimes it's used to stabilize banks along highways or streams, but it spreads out of control.

I saw some just today at the Morton Arboretum, believe it or not, where they were in process of stabilizing a stream bank with Illinois state funds. I don't know if it was intentional, but there was a big patch of crown vetch beside the bridge and sign explaining the work.

It is often planted by highway workman because it grows fast and forms a strong root system. It's a legume and kind of looks like clover, so it's nitrogen fixing. Its flowers are pink and purple balls, which you can see at the link below. Since you said what you saw was blue, you might want to take a look and see if it's the same stuff.

I like it, but don't think I'd recommend it. We had a small patch beside our front steps at a university rental cottage when we lived downtown, and thankfully it never escaped that bed. But I saw whole hillsides at Six Flags Great America covered with it one year when we went.

Here is a link that might be useful: Crown Vetch - Nature Conservancy

I think you are right, that it is Crown Vetch. It is pretty, but would surely overwhelm my little garden, where I thought it might be nice as a filler in beds with other plants. Oh well, at least I got that out of my system. Thank you for helping me.

Elisabeth.

Sweet woodruff or lamium might be similar alternatives.