There are members that will be able to provide better insight than I in this matter - but this is not normal in any way. Any cat that goes from normal size to anorexic size in a year has some medical issues.
If a blood test 4 months ago revealed nothing, it does not mean the cat is healthy if it is a walking skeleton. Has any other follow up been done?
Seems clear to me this cat needs additional veterinary care in a serious way and if this current vet is not concerned....I'd consider another vet that comes recommended from someone I trust.