Database: college_completion
Question: Calculate the percentage of Black students in all private for profit institutions.
SQL: SELECT CAST(SUM(CASE WHEN T2.race = 'B' THEN 1 ELSE 0 END) AS REAL) * 100 / SUM(T2.grad_cohort) FROM institution_details AS T1 INNER JOIN institution_grads AS T2 ON T1.unitid = T2.unitid WHERE T2.race = 'B' AND T1.control = 'Private for-profit'