To be certain we would need a picture since I don't know what your idea of "looking like a lily" is. Afterall, if you were real certain it wouldn't be a "mystery plant", now would it? :)

But "Black Beuty" is in no way "scary" in it's spreading habit and I can't imagine a 4' plant having no buds at all. I'm 2 zones warmer than you and my Black Beauty have small buds on them. Did you look very closely at the top to see if any are forming?

But one good way to find out, especially if you have a lot of them, is to dig one of them up. Dig deep so you are sure to get whatever, root, tuber, rhizome, or bulb is supporting the plant. A lily bulb is pretty unmistakable looking.

The Royal Horticultural Society "peopagating lilies" page has a photo of a dug up lily with the large bulbs at the bottom and stem bulblets (offsets) along the underground portion of the stem. They also have a close up of a cleaned bulb and its scales.

Hope this helps.

-Helen
http://www.rhs.org.uk/Advice/profiles1001/prop_lilies.asp

Here is a link that might be useful: RHS propagating lilies page

Thank you, I am certain they are lillies now after checking the growing pattern of them as they are pretty much in a straight line and all of about equal height. I do know what a lilly bulb looks but so far as me saying they were lilly looking.. The plant itself looks exactly like a lilly that hasnt gotten its buds yet but I had never seen such a large lilly before.

A picture would really help. I can not imagine 'Black Beauty' spreading in the manner you describe. It would rather look like a tall lily surrounded by smaller babies, not equal height plants in a straight line.

I have Black Beauty in Zone 5 and mine right now is 5 feet tall and forming buds now about half an inch long.

I'm late to the game, and I'm sure that the question was answered when/if they flowered, but out of curiosity is it possible it turned out to be euphorbia lathyrus? I fell victim to this fella once. At least in my zone they frequently "volunteer" and will grow in a straight line (for instance along a fence). Thinking they were cool, unusual looking volunteers, I tended them carefully, and they got to about 5ft before spreading out at the top with a Dr. Seuss like head of small yellow flowers.

Before developing flowers, I can see how they would look sort of lily-like (mine had stalks the diameter of a quarter), and although the leaves were lighter green, not grooved, and somewhat variagated, they were roughly oriental hybrid shape/size. In my opinion, these little fellas are hateful, but that's primarily based on my triffid-like experience of ending up in the emergency room after being attacked by one. Okay, so in reality the highly toxic and water resistant sap exploded in my eyes when I cut the stalk, but I choose to believe it was part of their plot to enslave the planet by blinding us.

http://www.botanypictures.com/plantimages/euphorbia%20lathyrus%2005%20NL%20uithof.JPG