I didn't see a new post for today, so I'll respond here, although it is usually best to not keep older posts going, so in the future, please start a new post for the day. If she is noticeably underweight, it may be a few things that could be responsible. Did your vet have any suggestions? I usually prefer to let the vet handle things like this, since a) I can't see Chloe, and they are able to better assess her condition with a proper physical exam, as well as labs and b) I'm just a regular person using a little of my own experience to try and help

, so just keep that in mind.
As I posted above, Chloe may not be getting enough calories each day, perhaps one of the other cats is eating her food, without you realizing? For a 10 lb inactive cat, you'd need minimum 218 calories a day, generally speaking. So with the 9-lives, depending on the variety, you'd be in the range of 1 1/4 can a day. Is that how much she is eating? You have noted she is underweight, and if you are feeding about 218 calories a day, and the other cats aren't getting into her food, then you may want to bump up her food intake to get at least around 260 -300 calories a day to see if that helps put weight on.
I can't speak to why she is losing weight, yet the blood work is normal, since I don't see them posted in your SS, but did they run a thyroid test (either a T3 or T4)? I think the T4 does come with a senior panel. Was that normal? The other thing you may need to rule out is intestinal parasites, especially if she goes outdoors.
My first thought was that she wasn't getting enough calories, but I'm adding some other things you may want to run by your vet, although I would hope they came up in your vet visit, but if they didn't, I'd ask just so they can be ruled out.