The fresh young leaves of most Ribes species are said to contain small amounts of hydrogen cyanide and have the potential to cause serious problems if eaten in quantity. Although we can find no cases of tortoises being poisoned through eating these leaves we would advise erring on the side of caution and not deliberately feeding this plant (although there is nothing to worry about if your tortoise happens to nibble a few leaves that fall into his enclosure).
The fruit of the Black Currant should never be fed unless your tortoise is one of the fruit-eating species. The pretty bell-shaped flowers of Currants are usually greenish white or cream and sometimes flushed with the palest pink. See also Flowering Currant, White Currant, Red Currant.