How many All Star players who played in the 1973 season were black?
1973 season refers to season_id = 1973; black refers to race = 'B'
SELECT COUNT(DISTINCT T1.playerID) FROM players AS T1 INNER JOIN player_allstar AS T2 ON T1.playerID = T2.playerID WHERE T2.season_id = 1973 AND T1.race = 'B'