Scrape off as much tar as possible with a razor blade scraper. Be careful not to dig into the wall and keep the wall surface as smooth as possible.
Clean the remains of the tar-stained surface with an abrasive sponge dampened with kerosene in one hand while blotting the runoff with an absorbent cloth in the other hand. Allow the cleaned wall to dry completely.
Paint the cleaned wall according to label directions with a stain-blocking primer, available at home improvement stores. Let the primer dry completely.
Use a paintbrush or paint roller to apply a coat of latex paint to the wall. Let it dry for at least one hour before applying a second coat.