Write SQL query to solve given problem: Among the postal points in the District of Columbia, how many of them have an area with above 20000 black population?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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