Personally, I would remove the plant and all of its offspring. It is rapidly becoming an invasive nuisance in many parts of the country and is on the noxious weed list of a number of states. It's a rather weedy looking shrub under the best of conditions, contrary to common belief, not the best butterfly attractor (native plants tend to far supercede this attribute) and you have seen one of the reasons for the growing problem this plant poses - it reseeds freely and with great abandon. A single blossom, if not deadheaded before the seeds ripen and disperse, can contain as many as 40,000 viable seeds. There are so many other more attractive yet less troublesome plants available to ornament your garden than to choose one which can and does overtake natural ecosystems. Buddleia gone bad indeed!

Laura, I've had a Black Knight for several years now and I have yet to find a single seedling anywhere.

First of all, I doubt you have a Black Knight there. I have two of them, one in full sun, one in part shade and neither of them have ever had a yellow bloom, nor have they reseeded themselves. Both bloom a deep, dark purple.

I also don't prune in the spring, rather I do it in fall and only very lightly. If you prune in Spring you are going to remove the buds that developed over the winter. I made the mistake one time of pruning buddleia in Spring and will never do that again.

If you don't want a yellow buddleia, you ought to just remove it and all it's offspring and compost the remains. And not to suggest that your plant supplier isn't reputable, and recognizing that sometimes mistakes are bound to happen, but I might be inclined to shop elswhere in future.

Cheryl

Sorry, but buddleia blooms on NEW growth - you can cut it back nearly to the ground each season and it will still bloom. In colder climates, like the OP's Michigan, fall pruning is not recommended as it can affect winter hardiness. 'Black Knight' is not a sterile cultivar and it can very easily set seed, regardless of whether or not you actually see seedlings nearby. In my area, butterfly bushes appear springing up in cracks in parking lots and alongside the freeway, nowhere near existing plants - the seeds are carried by wind and birds. And the question is pretty much moot anyway, if the OP has already seen seedlings emerge, regardless of what cultivar his/hers is.

I doubt reseeding will be a problem in Zone 5 Michigan. I dont't think the season is long enough for it to set viable seed there.

Maybe in Texas you can prune in the fall, but we have soggy winters and springs here in Maryland. If you prune in fall, you are likely to end up with your butterfly bush rotting before spring. I always wait until early spring and I always have flowers within a few months. I get lots of questions from people about why their butterfly bush died, and invariably they cut it back in the fall instead of early spring.

Seeds like crazy in Zone5 NY.

Ever wonder why Buddleia & Caryopteris are usually in the perennial section? It's because they are grown by perennial growers since they only barely qualify as woody shrubs. They are grown from cuttings before they start to bloom therefore easy to get them mixed up in the prop house.
Just about any shrub that blooms after the Summer Solstice is blooming on new wood.
Buddleia davidii is a Chinese native. It seeds itself in dry, rocky, waste areas. Take a drive up I-83 from Baltimore to Harrisburg any July. Outside of the guardrail is thick with butterfly bush in bloom. We have a granite quarry near us in NE Maryland, the unused parts of the quarry are covered with it.
Butterfly Bush is a staple in the Summer months which drives consumer interest like few other shrubs. If deadheaded it will continue to bloom. Now if we could only find a sterile form.
Sam

OK, I stand corrected - a friend in the Baltimore area tells me she finds little butterfly bush babies all over the back of her property. Didn't ask if she had any Black Knights, though.