I've not heard anything about poisoning cats, but green tea sourced from China sold in the west contained the toxic pigment Prussian Blue, as discovered by Robert Fortune on his expedition to collect tea plants to start plantations in India. This was at a time when foreigners were forbidden in China, and he risked death by going.
Mr Fortune witnessed the process of colouring them in the Hung-chow green-tea country, and describes the process. The substance used is a powder consisting of four parts of gypsum and three parts of Prussian blue, which was applied to the teas during the last process of roasting.
'During this part of the operation,' he says, 'the hands of the workmen were quite blue. I could not help thinking, that if any green-tea drinkers had been present during the operation, their taste would have been corrected, and, I may be allowed to add, improved. One day, an English gentleman in Shang-hae, being in conversation with some Chinese from the green-tea country, asked them what reasons they had for dyeing the tea, and whether it would not be better without undergoing this process. They acknowledged that tea was much better when prepared without having any such ingredients mixed with it, and that they never drank dyed teas themselves; but justly remarked, that as foreigners seemed to prefer having a mixture of Prussian blue and gypsum with their tea, to make it look uniform and pretty, and as these ingredients were cheap enough, the Chinese had no objections to supply them, especially as such teas always fetched a higher price!'
Read the section called The Tea Countries of China in the file below for more of the story. http://www.gutenberg.org/files/20792/20792-8.txt