"Black sooty residue" is probably the key here -- I suspect what you have is actually called Black Sooty Mould -- a mould which grows on honeydew, which is the sticky-sweet discharge from sucking insects like scale or aphids. I would look carefully at the branches and undersides of the leaves, I suspect you will find an infestation of either scale insects, mealybugs, aphids, or similar. If so, I would treat the trees, either with an oil-type spray, or with a systemic insecticide like Imadicloprid. Dennis SE Michigan

I get that smell from the two I have planted right by my front door - I love it too! Mine are not particularly fast growing, though. I would think that if you're aiming mainly for the scent, it wouldn't matter if yours never get very large. One or two should do the trick.

Green Gem would also fit that requirement, with a roundc compact shape, ideal for trimming to a hedge form. We planted a row on our retaining wall. My initial impression vs. my own experience is they aren't as slow growing as you might think. The only problem I've experienced is the proximity to the retaining wall, leaving the roots in the winter more susceptible to cold, and had some browning on these as a result. Could also be lack of water retention in the growing months as well. They will be moved back to a more appropriate distance. I have Green Velvet in full shade, north wall (minimal late afternoon sun), it has grown considerably well, probably a wee bit larger than 3 x 3, a nice mound form. Due to shade, it's not as dense as others that receive more sunlight. Absolutely no browning on my green velvets. I have a Green Gem next to my driveway, and the reflection in the summer caused some browning. It needs to be moved, not the best location. And as much as I've read about the boxwood scent, I have yet to smell it. LOL. I'm biased because I love boxwood, but if I had to choose between the rhody and boxwood, I'd go with the latter for the impact. They're also great to display lights over the holiday season, if you are into that.

Thanks for all the replies, I really appreciate it, especially Phil for posting the pictures. I think mine is mislabeled; there is no variegation at all, only green. I bought it as a hosta garden collection through Costco online. I know I can return it for my money back, but I do like the other hostas that came with it. I know it is risky to buy it this way, but I have not been able to source this one locally. My Frances WIlliams bit the dust a while back and I had read that OBL was an improved version. For what it's worth, all the hostas are in great condition and are thriving under the lights right now, just wished they were more careful about their labeling...Grr.