The Negro Leagues were a successful black enterprise that prospered, especially in its second iteration from 1933-1948. Fans attended in droves to watch their favorite players compete in thrilling games.
Regular season games had steady attendance, especially after WWII. Exhibition matches, including barnstorming, were also very popular even pre-war. All-star games, while the least frequent, had the highest attendance.
However, the consequence of integration was that the Negro Leagues could no longer co-exist alongside the newly integrated MLB. As players left, so did fans, ultimately depriving teams of the needed revenue to continue.