Hi there! Based upon the research I've done! It's most likely that the people has believed that blacks weren't ever allowed the privilege of the quote "once free, always free". But considering that Dred Scott was sold in Wisconsin *A Non-Slave State*, he met Harriet and soon they had a family and so they've filed a freedom paper and later went to the supreme court.
So it's obvious that the whites' however thought that the blacks never were allowed such privileges and the blacks who were slaves must've tried they're best as well to become free or to free their young ones at least.
Hope this helps and have a fantastic day!