Among the postal points in the District of Columbia, how many of them have an area with above 20000 black population?
District of Columbia refers to county = 'DISTRICT OF COLUMBIA'; postal points refer to zip_code; area with above 20000 black population refers to black_population > 20000;
SELECT COUNT(T1.zip_code) FROM country AS T1 INNER JOIN zip_data AS T2 ON T1.zip_code = T2.zip_code WHERE T1.county = 'DISTRICT OF COLUMBIA' AND T2.black_population > 20000