Black people would've boycotted the Harriet Tubman movie back in the day if a studio exec got his wish for Julia Roberts to play the legendary abolitionist ... so says Harriet's relative.
Harriet's great-great-great-grandniece, Tina Wyatt, tells TMZ ... the movie would've been blacklisted in the black community because it would've been a slap in the face, and utterly disrespectful if Julia were cast to play the black woman who helped free countless slaves.
As you know ... screenwriter Gregory Allen Howard claims a Hollywood exec listened to his movie pitch for a Tubman project more than 25 years ago and told the room Julia should play Harriet. Howard says someone pointed out the fact Tubman was black and Roberts was white, and the exec said it was so long ago no one would know the difference.
Harriet's relative says it's insulting the exec would think black people would be so ignorant as to not know their own history.
The Tubman movie is finally in theaters -- with a black woman playing Harriet -- and Tina thinks white folks should check it out and brush up on their history because she's noticed an alarming trend.