I am not a vet but based on personal experience with a blockage in one of my cats it is very possible for a blockage to happen without crystals or stones in the urinary tract. My cat had a blockage caused by a nasty UTI that went severe in less than 24 hours before showing any symptoms. Two different vets told me the UTI caused the blockage and there were no crystals present. The lack of crystals/stones was verified by an ultra sound. Both vets told me that there are usually signs of the UTI before this happens but in this case the signs were just not there. My cat was not straining or having blood in his pee until he had already partially blocked. There we no pee accidents and he was not crying in the litter box before he partially blocked. I now watch this cat pee a couple times a day to make sure things look normal. Both the ER vet and the vet I got a 2nd opinion from that hospitalized my cat said this is not the norm for how a UTI presents but it does happen.
