This is an Asiatic Dayflower, according to my Wildflowers of North America field guide. It is one of the very few really blue flowers - not a shade of violet, but blue. These dayflowers are rather small, about an inch in width and each flower only last for a day.
I have several small patches of these around my yard. They may be weeds, but how can you call them weeds when they have this truly blue flower.
This drawing is in my sketchbook and was done with prismacolor colored pencils.