To change the background, no. You need a layers editor to do that (such as Photoshop or Affinity Photo).
To blur the background. Not something I would attempt in LR, others might chip in with how they would go about it. Again, a layers editor job, if you want to do it well.
To darken the background, possibly. LR has a develop module tool that will darken (or lighten) an oval shape. Go to Develop>Effects>Post-Crop vignetting. Move the 'amount' slider up and down. The other controls vary how 'round' the shape is, how quickly the effect is transitioned in (feather) and a few others, try them to see what they do.
BTW, I use 6.14, so far as I am aware the latest versions of LR have not changed this capability significantly.
While LR can make local adjustments, it isn't really much good at it, especially for large and complex shapes. Layers editors work in a very different way. I prefer to think of LR as a raw editor, making changes to the whole image. Yes, I'll occasionally fix a dust bunny or other local adjustment, but it quickly becomes painful (horribly slow) in my experience. Layers editors are much, much better at such changes. Use LR to get the whole image as close to right as possible and then work in a layers editor. I might add that I rarely use a layers editor, I find the processing commands in LR are entirely adequate to convert raw data into a publishable final image, but then, I'm not blurring out the background etc, I aim to do that in camera.