I am so sorry to hear about Blackie
Do you have his readings from tests cos this will give you an indication.
What did the vet say, is he dehydrated, is he eating, has he lost a lot of weight very quickly and what age is he?
I know they can go down very suddenly and lost Ducha my 19yr old last Aug and although he had kidney probs for quite a long time although never treated cos he couldnt be pilled, he literally went down over night and when I took him to the vets within 24 hrs on what was sadly to be his last journey, Gareth told me they could put him on a drip but that would only last for about 4 days and we would be back to square one.
This is a discussion that you can only have with your vets really and ask him what he would do if it if it was his cat. It really depends on so many things.