Blacks are reacting with shock and anger, calling for greater vigilance by police and schools against racism in the San Francisco Bay Area after a young white supremacist allegedly threatened to lynch a black woman. Kenneth Lamar Allen, 20, of San Jose was booked on felony charges of attempted robbery and false imprisonment. "We used to say racism is here but subtle," said Lurlene Bush, president of the San Jose branch of the National Assn. for the Advancement of Colored People. "Now we can say it is here but it is not so subtle."