Hyacinthoides

Oh, you must be right. That's *not* what I was thinking of...I was thinking of a ground cover I can't remember the name of that ends in repens (no, not dichondra - it had purple flowers very like this one). Well, so I suppose I don't want to plant it, then. It was growing among the forget me nots.

A close-up of the flower and of the foliage could be helpful.

Looks like tall garden phlox to me. I had a very hard time getting rid of mine because the roots go very deep. (I didn't want it in that bed.) As a consequence, it is very drought resistant and will come back each year.