This is a SF novella I read in the late 1980s in the UK.
There is a domed city where people went for refuge generations earlier, the predicted apocalypse never happened but they're unaware.
The outside world, now quite advanced, finally finds a way through the shield barrier and sends one person through, to both make contact and to prepare the lost society for culture shock.
The person, a woman, arrives in the middle of the night, so she has a walk around to pick up cultural clues, she comes to a scary conclusion so hides before daylight.
It seems the cut-off society has had a race war during its history and killed all people of colour, unfortunately the infiltrator is African American and will be slaughtered on sight.
She has a year to survive before the outside people stop the dome shield to make contact, they assume that, by that time, a fully prepared society will be keenly waiting for them.
All she can do is creep around at nights and leave bits of graffiti to try and prepare/enlighten the dome peoples. And hope she can last the twelve months.