Symphytum azureum (The common name (comfrey) is very misleading. This is NOT the comfrey that most people know.) It is one of those plants that is both exceptionally useful and a nuisance to get rid of. I am just coming off about a ten year honeymoon with the plant where everything it did pleased me, so I am surprised at how little is seems to be known and offered. Curiously, a Google search brings up mostly sites outside of the U.S. An issue over its proper name may be one reason.
It spreads slowly. It isn't what you would call invasive. Inexorable would be a better description for its slow but relentless spread throughout a suitable growing site. So now after a decade it has reached the boundaries of where I want it to be, but I find it does not come with an off switch. It would not be very compatible with mixed perennial plantings. I fear it would insinuate itself into most anything herbaceous, although I imagine larger plants could readily grow through it. By the way the web site for the German nursery Lorenz von Ehren says that prompt deadheading will slow its spread.
What has been exciting to me is that it is both attractive and very successful at developing a dense stand under the heavy shade of a sugar maple and a massive Norway spruce. For the very patient or the free spender it makes a great groundcover for the shade. The catch is that it must be contained (or perhaps diligently deadheaded).
On the whole it is a very useful plant that would bring considerable joy to gardeners, especially where an attractive solid stand of a highly shade tolerant groundcover is wanted.