In my mind I'm trying to turn these black blobs into silhouettes, but I think that this really points to a need for editing practice. To get a background darker, first use either brightness/contrast or levels to adjust it as much as you can. Then use your burn brush or a black colored paintbrush set to a low opacity (maybe 20%, depending on the situation) and low hardness, and layer in the darkness so that it blends more evenly. Right now I can see exactly where you darkened, and this smoke shouldn't have any hard edges. By the way, a plain chunk of black velvet works beautifully as a black background, it reflects virtually no light. They actually used to use it before they used bluescreens, because when photographed it looks almost pure black. I finally bought a yard of it at the local fabric shop, and I do much less editing now!
It's a nice quiet image with a good flow. Black background fits nicely. A title "untitled" shows little creativity- find something that fits. "UP, UP and Away?" "Silver threads?" "Cosmic?"
I can see the hard lines of your selection around the smoke, adn that foils the effect. Use of levels, or curves would make the desired effect easier i think, and from the look of this you also need a deeper depth of field - very little is in focus - or you need a faster shutter speed - if it's motion blur. I know from my own experience that it's very tricky with smoke ... but my sympathy for your attempt grants you a few points.