So sorry you are going through this and thank you for caring.
If your vet is not an avian certified vet, I would strongly suggest that you try to find one. An exotics vet that sees parrots is not the same as one with true avian certification.
My guess: PDD.
A normal blood test won't detect it--- it is a special panel that must be run separately and may involve swabs, x-rays and biopsies. Also-- FALSE NEGATIVES CAN AND DO OCCUR and
up until very recently, there wasn't a test for it (other than post-mortem).
That is what is sounds like it could be--I am not a vet, but it's neurological and leads to seed in poop. It is also horribly contagious (but thankfully,
does not live on surfaces for years like some avian viruses) BUT you should not bring any other birds into the home for now and any you do already have should be strictly quarantined. Similarly, if you contact her bird and then other birds, you should be changing clothes etc before contacting them (and all birds who have been around this one should be considered at risk, even if they show no symptoms). Like I said, it is very contagious---the silver lining is that it won't last for years like some in carpets/H-VAC system. Birds can carry and transmit ABV for a lifetime with no symptoms, but if their immune system is weakened due to stress, age etc, it sometimes progresses to PDD. You want to avoid spreading either, as the ultimate result of ABV is PDD (if it progresses).
As you know, if it is PDD, it is degenerative. PDD stands for Proventricular Dilation Disease and is sometimes called "Macaw Wasting disease" (I may have spelled that wrong, but Google will correct it if I did)
ABV (Avian Borna Virus) is generally though to be the precursor to PDD--sort of like HIV and AIDS. An ABV positive bird can live for a long time and pass it around with no symptoms, but if it progresses to PDD it is worse for the bird. There is a lot that is still unknown about this virus because it is pretty new in comparison to some parrot illnesses.
I typed all of that from what I have learned over the years, but I wanted to confirm it and the following link from VCA seems to do so...AGAIN, I am not certain, but that is what it sounds like:
https://vcahospitals.com/know-your-pet/proventricular-dilatation-in-birds
Here is another-- it mentions changing shoes and washing between bird areas:
https://lafeber.com/vet/avian-bornavirus-primer/
Here is another link (please note-- you should never use bleach in a home around a parrot--- these cleaning methods would have to be conducted AWAY from all birds in the home):
https://www.wikihow.com/Treat-Macaw-Wasting-Disease <--also, please remember that (much like COVID-19) even a perfectly healthy bird can spread and shed the virus. Unlike Covid-19, this transmission can be indefinite (e.g., more than 50 years in some carrier cases). Boarding etc should be discouraged if a bird has been in contact with a suspected infection.